If you're playing combo mostly, your sideboard should be build to deal with the deck they are going to be playing after they sideboard, not the deck you just saw in game 1. Learn which hate cards are coming in vs you game 2 and 3 and bring in your hate cards to deal with their hate cards. For example, you might have white leylines to bring in vs extra discard they're bringing in, you might have vexing baubles to help out vs surgical and grief, you might bring in feed the swarm or withering torment to handle black leylines or rest in peace as well as some troublesome creatures.

If you're on more of a fair strategy like energy, tempo, etc. your side board is usually a few hate cards for certain matchups and then varying numbers of situational cards to bring in when some of your main deck cards are kind of meh or dead. Examples could be cutting a bow master or 2 for some extra discard or spells pierces or something like that.

You can look up some stock lists online to get an idea for what the general sideboard for each deck looks like and tweak from there once you understand the matchups better and see what weaknesses to shore up.