LevelASM inserts a specific ASM code on a per level basis. Such as a HDMA gradient for one level or the example code found under Level 105.
The way you add it is by just patching level.asm
, setting the freespace and all - and just placing the codes you want in the level code or level INIT ASM files, under the level number you want it to be in.
Depending on want you want will depend on which you put it in.
In case you don't know how to apply patches, there's this
There's also UberASM but I've never really used that as it came out after I started with LevelASM in my last hack.