What type of scrapbooking software are you using? That might help someone to answer this for you :)

With my Scrapbook Max, I can right-click on the item and select 'move to back' or 'move back' (which would take it back one layer at a time), or 'move to front' or 'move foreward'.

If you are using Scrapbook Flair software you will find your "layers" on the right side. Just slide the one you want on top, to the top. You can do this with each piece if you want them behind or in front of another piece. If you can't see your column of pieces on the right, look for a tiny black arrow around the middle of the right side of the workspace. Click on it and your column will appear.
Hope this helps.