So, quick question: Do you know how to look inside a goomod file?
If the answer is no, you have to rename the .goomod extension to .zip. If you don't know how to do that, look it up. Looking inside goomods is an absolutely necessary skill for making complex mods like chapters. It's also just handy in general because you can see how mods work.
If the answer is yes though, then great! Below is a list of steps you'll need to do.

1. For each level you want to include, you'll want to make a goomod file. That way, you'll have all the files you need for the level pack.
2. Now that you have all the files, you'll want to combine all the compile folders together, as well as all the override folders if there are any. At the end, you should have a compile folder and (if applicable) an override folder, both of which contain everything include in each individual level.
3. If your goomods have text, then you'll have to add all the strings into one text file. You can do this by using one text.xml file, and copying the strings from the other text files into that one text file.
4. The last step has to do with the addin.xml. Notice how there's block? Within that block, you'll want to copy the block from each goomod and paste it into that one block. The result is a block containing a block for each level included in the pack

Once that's done, you'll have all the folders and files you need to create a goomod (just select all the files and folders, create a zip file, and then rename it into a goomod). If any of the instructions were unclear, I highly recommend you go download a level pack and see what they look like on the inside. It's extremely helpful.

ok. thanks very much