There are I think 2 different issues here:

1. Do we really need to go to Mars, and if so when?

Instead of, for example, devote more funds to unmanned space probes, or devote more money to solving the problems of humanity on earth

Economic reasoning/and what purpose (just scientific, or prospect of some independend colony)

2. How should we do it?

A sized up Apollo project, or a gradual approach? (first dozens more robotic missions, robotic return missions, robotic missions that build life systems (mainly oxygen, water, food crops, shelter) and only after that attempt some human mission).

As to 1

I don't think it is very feasible and realistic to do it now (let's say within a time frame of 2010-2030), there are huge technical difficulties, it costs astronomous amounts of money.
The science community is not in favour of it. Let's face it: Apollo was for political reasons (prestige of the USA, beating the Soviet Union in the space race as moral prestige), and much less scientific. Those reasons do not exist now to do something similar, so no funds.

If the reasoning is that humanity is better off having settlements on 2 planets, then please realise that the only big threat we have is ourselves, and not some catastrophe from nature we would not survive (at least not on astronomical short scales). If for example our biggest concern were for example astroid impacts (as that is a possible threat to all life on earth), we could built a defense system or built settlements in locations that offer protection (why not built settlements on the ocean floors? relatively safe! If we can not do that, how then could we built stable settlements on Mars??)

So, in this perspective, I think there are no urgent reasons to go to Mars now, or in a small time frame.

And the realistic costs for having a real settlement on Mars (not just a couple of return trips) are enormous, since for a long period that settlement will entirely depend on resources from earth, and the transport costs are enormous. How to built a self subsistent community on a planet like Mars with no eco-system, no fuels, little sun-light, scarce water supplies, etc.?
If you make calculations about this in economic terms, this project will have enormous costs, and no clear indication it can ever break even.
For whatever reason there is to do this, I guess that all alternatives for that are simply much cheaper.

Esp. the reasoning some human-Mars-missions that settling/colonising Mars for the reason of having more living space and resources and to provide a solution to earth/humanity most urgent problems (depletion of various earth resources, like fossil fuels, drinking water, food crops/life stock, different other resources), definitely going to mars offers no solution, instead will worsen the problem since such a colony will be parasitic for a long time. There is not much to start with on Mars to build upon (not an eco system that existed for billions of years, no infrastructure left behind by previous civilisations), you would have infact to start from scratch, except for having a lot of technological knowledge.

So, my conclusion would be that we should not refrain from normal economic reasoning in the context of space settlements, and temper our enthousiasm for going on a human mission (at least in the prospect of some permanent settlement) very soon (i.e. before 2040 or so).

For the sake of the argument that 'some day' we need to have a different location as the sun turns red giant (the one catastrophe foreseeable that not even with future technology would be solvable, except for translocation to Mars or moons of the gas giants), the time frame for that allows a little delay of the Mars mission. If we make a most pessimistic that this events urges for leaving earth in some billion year from now, I guess this leaves us some time to think for a solution.

Some very rough calculation, let us divide this gigantic enterprise in some smaller steps:

Step 1: Be able to land on mars and return (robottic mission)
Step 2: Be able to land on mars and return (human mission)
Step 3: Be able to permanently settle and be self-sustanained on Mars
Step 4: Be able to translocate large portion of humanity to Mars
Step 5: Be able to land on an extra solar planet (robotic mission)
Step 6: Be able to land on an extra solar planet and return (robotic mission)
Step 7: Be able to land a human mission to an extra solar planet and return
Step 8: Be able to permanently settle on an extra solar planet and be self-sustained
Step 9: Be able to translocate large portion of humanity to an extra solar planet

Let us make some estimates here, by estimating how much more difficult each step would be as the previous, and so how much longer it would cost to accomplish as the previous step.

Amount of time necessary compared to previous step:
For step 2: around 2-5 times
For step 3: around 10-100 times
For step 4: around 100-1000 times

This reads like: if we can have a return robotic mission to mars in 10 years, then we can be all on mars in like 20.000-5.000.000 years from now.

For step 5: around 2-10 times
For step 6: around 2 times
For step 7: around 2-5 times
For step 8: around 10-100 times
For step 9: around 100-1000 times

The lower estimate then for translocation to an extra-solar planet is:
16 million years to 5 billion years

Of course these are very wild guesses, and are based at the assumption that scientific progress advances steadily, and the technological barriers (most importantly the rocket technology that can deliver speeds nearing at least10% of lightspeed, else we can't get nowhere) can be overcome.

Some wild assumption in this is that rocket science progress allows for 10 times faster traveling in a time frame of 100 years (this rule will not apply when relativistic effects have to be taken into account, so above 1-10% of light speed the technological impact on increase of speed will become less). In this scenario a one-way robotic mission to Proxima would be possible at speeds of:
In 40.000 years: 10000 km/s (0,33 % of light speed; single trip Proxima would cost approx. 2600 years)
In 5.000.000 years: 100000 km/s (3,3 % of light speed: single trip Proxima would cost approx. 260 years)

So, I guess in this timeframe it is possible to "save" all of humanity from excarnation before the sun is red-giant (even in the pessimistic scenario, since we will have different places in the solar system already were humanity can live before the effects of the run becoming red giant become a serious threat to humanity (which will perhaps start in 500 million to 1 billion years from now).
Even slower time frames would make it possible for humanity to survive, if we can translocate to cooler places in the solar system (moons of Jupiter), but for best survival conditions, a suitable planet in an exa solar system (and probably different planets for most survival probability) would be much better of course.

So, this kind of scenario is the only thing we seriously need to consider in planning for human mars missions. It shows that there is no urgent need to hurry, but neither can we delay it too long.
So, a human mission to mars within 50 years seams a reasonable time frame.

As to 2

What would be the clever way to do it?

I don't think a scaled-up Apollo mission will get us there. We need to think in smaller steps:
First some more robotic missions, to have some more detailed info on Mars, thereafter robotic return missions. Both for developing the return stage techniques and for more closer examination of Mars samples.

Second, we can first develop robottic missions that can built life support systems, so that oxygen, water and food crops can be available for long mars missions.

The third stage is to built a permanent station in Mars orbit (like the ISS). First components can be brought by automatic missions. Thereafter (optionally) permantly manned by a crew of 4-8 people, and resupplied at least 4 times a year. The station should be able to function automatically for several years.

Then we can start a series of trips to Mars. First for doing some more exploration, but then also to built equipment for later missions and in preparing for a permanent Mars ground base.

The most difficult parts in this are the landing and return stages (Mars has far more gravity then the Moon) and landing is also exceptionally difficult with a thin atmosphere, and how humans can accomodate for such long trips for a large part in low gravity, radiation risk, etc.

Of course, if space elevators were develop and in operation around that time, the expensive trips from earth orbit would become much more economical. If the concept of space elevators is possible, perhaps such ventures should wait for the realisation of space elevators first.