One type of content you will find in this forum is the NPC pack (or Block pack, or BGO pack, or Effect pack... maybe even a mix! For simplicity I will just refer to NPC packs however).
Using these in your own levels is simple. The contents of such a pack might look like this:
You can extract all the contents of a folder like the one above into your level/episode folder and the corresponding IDs will be automatically applied. In this case, 751 and 752.
All IDs between 751 and 1000 for NPCs, blocks, BGOs and effects are available for per-level or per-episode customisation. So if you would like to use several packs, but there are overlaps in IDs you can do the following:
1. Rename the files of the secondary pack you are importing to fill IDs currently not occupied. Make sure to stay within the 751-1000 range.
2. Open the npc-n.ini file for the NPCs you have renamed, to make sure they use the same image file as their name suggests.
Afterwards, if you have the editor open, hit F8 to refresh and the new content will be available! Depending on the renaming situation you might have to adjust a few tilesets (if the pack creator provides them) or create your own in order to access them in the Tileset Itembox. Simply checking the "Show Custom Only" box and selecting the type of object you want to create a tileset for will help you easily find the things you have imported:
Not all npcs you'll find here are 100% coded from scratch. Sometimes you may find adaptations of basegame code, or adaptations from other people's code. Crediting these properly can be a bit daunting if sufficient information isn't provided. In the case of a basegame adaptation (e.g. custom mod for Thwimps), credit the modder as well as the original creator, found in Credits.xls of your SMBX2 installation. In the case of a mod of a user-submitted NPC, please find the original author in the original post by the modder. If the modder didn't provide this information, please remind them to do so. Thank you!
And that's it! You can now start using packs you have downloaded here in your levels. Please enjoy!