Find the collection you created previous step in collection table, then copy the id column value and paste it into a temporary file.
Find the all project you want to add to collection in project table, then copy the id column value and paste it into a temporary file.
Add records to project_collection table.
The project_id column value is the id column value you copied from project table, the collection_id column value is the id column value you copied from collection table.
You can also open your database provider's dashboard to do these