Lumping Decorates together

I have about 15 decorate monsters and weapons that I downloaded from the web, and I need to lump them together into a single wad file.

What approach should I take?

What, specifically, do you mean by "approach"? Do you mean the file format (e.g., .wad, .pk3)? Do you mean the tool to assemble everything into a package (e.g., SLADE, XWE, SlumpEd)? Do you mean the method to pull everything together (e.g., creating a DECORATE lump, assigning Spawn ID numbers, assigning DooMEd numbers, putting your sprites between appropriate markers)?

I'd go one by one, copy/pasting entries and any DECORATE into your compiled WAD. Make sure you test after each new WAD you add to your compiled wad. It's progressive you can't do it all at once since there would be a fairly high chance of some sort of collision in the data happening.

I always do each new download one-by-one. Copy all of the sprites, then sounds, then copy the decorate information into one big one in your wad.

IMPORTANT! Make sure to update SNDINFO, GLDEFS, etc. as necessary! Also, make sure no two decorate lumps share the same ID number.

Hope this helps!