A good exposition of the random restriction method
A relatively simple setting to illustrate the method of random restrictions is Subbotovskaya's original application of the method to prove an $\Omega(n^{1.5})$ lower bound on the formula size of the parity function, where formulas use arity-2 AND and OR operations. Section 6.3. of Jukna's Boolean Function Complexity has a nice exposition. Also, see these lecture notes from Swastik Kopparty's class in Rutgers.