(I'm not sure if the process is the same for scarlet, but if it is it will turn into armarouge)
First you need to catch a charcadet, google says the easiest area to get it is east area three
Next, you need to get to zapapico and wait till nighttime, then you have to either catch or defeat sinistea, and get ten of the material it drops
Then you find someone in zapapico standing in front of a fountain, trade the ten sinistea parts with them to get the item you need
And finally, use the item on charcadet, and now you have ceruledge
The item is consumed when you use it, so if you're trying to make an entire squad of ceruledge you're gonna have to do a lot of farming
For me the hardest part was getting charcadet, farming sinistea isn't hard it just takes a while