Go into steamapps/workshop/content/976730/3150191336/maps select all the files in that folder, copy and paste them into steam apps/common/halo the master chief collection/ halo3odst/maps but make sure to create a back up of the files before you paste the previously mentioned files.

Doing this will make it so whenever you progress to the next level instead of going back to the vanilla campaign you’ll play on the next level with the mod active