Well its not that hard, the side deck is basically a combination of 15 cards. Those 15 cards are meant to stop other current strong decks or decks you have somehow troubles with. So you will build your side deck based on what cards you think can be sided for a lot of decks, thus not wasting side deck space. Side decking opens a lot of options, and its really important to do it when playing a match. Thats a really brief explanation but thats all I feel like typing lol, go on google and type stuff like :"Side decking in yugioh" or "how to side deck", stuff like that, I am pretty sure you will find something to help you ^^