To add a drawing to one of your collections you must click onto the drawing which you wish to add. On the left hand side of the drawing which you've clicked on you should see this:
Click on the fourth one down which says 'add to collection'.
Then you will be directed to a page which shows you all of the collections which you currently have made. You just need to click on the collection which you wish to add the drawing to and it'll be added.
Hope this helps.
I am trying out my christmas present a bamboo splash and I drew an item to go into a collection, but how do I add the drawing to it?