In your head, try and use one equation to solve the problem. If you have too many unknowns, then try another equation. Eventually you'll find the one that works, hence you can solve the problem.
i always write down the values that i'm already given; then i write down a question mark next the value i'm trying to find. Then i just try and think of the equation that uses the values i have/need. But yeah, sometimes the question gives you enough information to use more than one suvat; in which case i just pick any
Write down "SUVAT" then next to the thing you need to find, put a question mark. Fill in other things given to you in the question (remember, if it tells you something starts from rest then u=0 and if it is falling down then a=-9.81) cross out the remaining letter. Use the equation that has all four letters shown.