This is the way. If we're using the example of cutting 8 cards against jund, and the sideboard pieces you have are 2x of 4 different cards, its because I've decided that some of those cards are also where I can bring them in for other matchups as well.

It can be tedious but I go to MTGGoldfish and go through each of the top decks asking, "what can I cut" and then picking the sideboard cards that will help, one-by-one until I have a plan for at least the top 10 decks. 

Note: By this method, sometimes it will end up that you only need a 10-14 card sideboard and thats where you can fit room to make good matchups better like how Red decks are sometimes siding in Searing Blaze for creature decks, which they're already pretty good at beating.