Put it on backwards:
- Flyline to reel arbor, followed by running line, then backing. When you've filled the reel or put on as much as you want, take it off and put on in reverse order.
- What Jeff Bandy said above.
I even had a fly shop put backing and line on a reel for me and they guessed the amount of backing, spliced it to the fly line(quite an elaborate splice by the way) spooled the fly line and didn't have enough room.They missed the fact that the line was 110 feet instead of 90..........had to redo the whole thing.