First, you need to dampen your hair either by spritzing it with a spray bottle or running through it with wet hands. Either way, just make sure your hair is slightly moist so that your curls can set when it dries overnight.
Second, there are many different methods, but here are the ones that worked for me.
Instead of just braiding two pigtails, I like to incorporate either a really long sock into each of the braid to make the braid thicker. Thus, loose curls rather than tight crimps. If you don't have long socks, just use old leggings.
Fold it in a half and use it as two of the strands in your braid. You could also do the rope braid (,you should probably look up a tutorial on how to do that if you don't already know,) using the sock/leggings/whatever you decide to use as one strand and your hair as the other in the braid.
This was inspired by Lilith Moon, who did a tutorial similar to this on Youtube. She suggested using a scarf or hair extensions and she did an entire French braid. She also has other tutorials for heatless curls.
I've also wrapped my hair around a sock. (I'm not talking about a sock bun.) For this, you would need a sock that goes past your ankles. I just part my hair in halves and bring them to the front. Then, starting with the ends, I wrap my hair around the (horizontal) sock. Once my ends are tucked in, I continue rolling the hair onto the sock and end it by tying the ends of the sock together. (This is why you need the sock to be long.)
You don't need mouse or serum necessarily, but you can still use it. I recommend hair spraying when you take you hair out in the morning, but I don't usually use hair spray because I don't like the stiff crunchy feeling and the residue it leaves.
You should definitely check out BeautyKLove, a Youtuber. She makes videos mainly about hair (she has drop-dead gorgeous long hair) but also expand to other stuff. She has plenty of videos full of creative ways to curl your hair heatlessly.
Otherwise... I hope this will be enough... ;)