That is a very vague question Im afraid, as there are many methods you may have to use, depending on what kind of formula. It sometimes is even impossible.
Assuming you mean something simple, basically if we want to isolate v, we take all the things with a v to one side of the formula, and everything else on the other side. If there are common factors, cancel them out. If it is still not plain v, say v^2, then we cancel that out with a square root, we've got v. But thats not a very good explanation. Give us an example and we'll tell you how to do that one.
To isolate a particular variable you must add, subtract, square root, power, divide or multiply variables around. Trying to cancel what's on the left or on the right.
Say you have
1/2mv^2 = mgh
To get velocity by itself. We first times each side by 2. This gets rid of the half.
mv^2 = 2mgh
Then we divide by mass and the masses cancel out.
v^2 = 2gh
Then we square root each side because it is v^2, and to get a powered number/variable by itself we must square root it.