For casual (i.e. Turf War) matches: just enter the queue and your friends can find your icon on the "Friends" option, and they can follow you from there.
For ranked matches: choose League Battle and "pair" or "squad", then your friends can follow you via "Friends". Note that you can only queue with 2 (pair) or 4 (squad) people. There's no option to queue with three people and having a random squid kid fill the last spot. Also worth noting is the fact that you can't play League until you've reached at least B- in any of the ranked modes' solo queue.