Is basicaly a shift register with taps a certain points being xor'ed and fed back to the input.There are some variations and the lenght of the shift register and the positions of the taps depend on the sequence length which is the number of shifts you can perform before the state of the shift register repeats itself.
Finding the positions of the taps to produce maximal lenght LFSRs involves handwaving and burbling about set theory but you can find precompiled tables.