I have a few mods and want to add aspects to the items.
Which method let me add aspects?

In class with Thauminocons methods:

public static final Aspect NAMEASPECT= new Aspect("nameaspect",color#hex, new Aspect[]{components(if your aspect have components)}, new ResourceLocation("modid/textures/aspects/nameaspect.png"),blend(int));

Sorry I meant add existing aspects to items

I have this code in Thaumic Additions:
https://github.com/Zeitheron/ThaumicAdditions/blob/1.12.2/src/main/java/com/zeitheron/thaumicadditions/init/KnowledgeTAR.java#L92-L167

It is exactly what you need :)