The following is merely a suggestion.
Personally I actually quite like the default setup of the M26, it suit's my particular driving style. However I know from experience that while one setup may work for me, it may not suit other drivers, so if a driver falls into this category i.e. either A. don't like the setup or B. cannot grasp how to control the car with all of it's understeer, and take advantage of it, All that I can suggest is to seriously spend some time in practice and dial the vehicle in to make the vehicle behave how You, "the driver" wants it to behave, Isn't this the purpose of a setup at its core to begin with?
If the setup is good, it builds confidence with the driver, and more confidence means that the driver can drive the car harder on the limit, faster, and ultimately, it would also be easier to drive.
You will need hundreds of laps to do this, and acute understanding on how the vehicle behaves.
The following below may help you. This is how I approach any new vehicle that i'm interested in making a setup for.
It is not an "ultimate" guide or anything, but this method works for me, and i'm freely sharing it out to you. Maybe it will help you understand the vehicle more.
Step 1. Get a pen and a notebook ready.
Step 2. Drive the default setup extensively, don't just drive one lap, drive 20, maybe 50 or more laps.
Step 3. Take the time to understand what your inputs do, relative to how the vehicle reacts.
Step 4. Drive the vehicle and write down in your book how the vehicle behaves in these three areas, Corner entry, Mid-Corner, and Corner Exit.
Step 5. Once you have experienced and identified the vehicles generals behaviour, go into setup and make one small change, and "write down the setup change you made" (IMPORTANT INFO: Do NOT change more than "One" value at a time. Otherwise you wont see what a single change did to the vehicle)
Step 6. Go back out on the track and drive the vehicle for several laps and note down how the vehicle behaves as described in step 4.
Step 7. Repeat Steps 5 through 6, untill you feel the vehicle behaves how you want it to behave.
You will also want to approach each circuit in the same manner, after you built your setup, don't be afraid to try the same setup on another ciruit, you may find that it works well, and you may also find that it doesn't, and if it doesn't, repeats steps 5 and 6 again for the new circuit.
While setting up a vehicle may seem very daunting at first, I can assure you, if you stick with it, it will pay off. I also find that making a setup might actually help you identify, or clearly define how the vehicle likes to behave. Not only that, it might actually help you improve yourself in the long run.
There are many handy guides out there that can tell you what each values do to the vehicle and what change is to be expected, I'm sure a simple google search can pull up plenty of guides. One guide that I use and would recommend would be the following: Setup Matrix
I hope this was informative and helpfull to you.
See you out there on the track!
And above all, have fun!