There's two bits of the mortal realms. One you get to decide on, one you don't.
The first bit is that you can claim your army is from a certain realm. In this case you can pick artifacts from this realm. You decide on this while building your list.
The second bit is that you can fight in a certain realm. This gives acces to the realm spells as well as some addititional rules that affect the battle like fireballs dropping randomly from the sky that sorta nonsense. There are no rules as far as I am aware of about how to decide this. So this you'l just have to agree upon with your opponent or ask the people at the tournament how they want you to figure out which realm it'l be.
To be honest, it's a shame you don't get the realm spells when picking what realm you're from. It'd help armies like ours, which have a lot of wizards, but no spell-lore, tremendously by making us perform more consistently as we would actually know which realmspells we have in advance so we can build around it, instead of adapting on the fly.