How to Backflush IV Tubing?

I'm not 100% sure if I'm answering your question right the way you're asking, but here goes:
you have your primary line already primed and loaded into the pump, hanging below the IVPB med. You hang the IVPB higher, but to prime the line, you should just be able to hold the bag and tubing below the main fluid running with the tubing unclamped, this should backflush it.
i do it all the time, but sometimes putting it into words is a little tough for me. you'll get it though!

Think about it: if you want to get fluid from one tubing to another, what would you do to both clamps??

i'm typing verbatim what it says in a book i had to get for school:
"1. ensure medication compatibility with primary infusing solution. Note: If medication is incompatible with primary IV solution, temporarily discontinue primary infusion. Flush clients injection port, initiate a normal saline as the primary, then proceed with 'piggyback' into the 'new' compatible primary. When complete, restart original primary solution.
2. spike bag with secondary administration set. Affix needleless cannula to end of secondary tubing.
3. cleanse injection port of primary tubing with antimicrobial swab.
4. insert needleless cannula of secondary 'piggyback' tubing into primary tubing, port above pump.
5. hang the secondary bag on the IV pole.
6. use extension hook to lower primary bag below secondary bag if indicated.
7. clear tubing of medication bag by opening clamp, temporarily placing secondary bag lower than primary solution bag and allowing primary solution to flow retrograde into secondary bag tubing (back priming).
8. Backfill until secondary tubing chamber is 1/3 full. Clamp secondary tubing. Rationale: this ensures no medication is lost during priming
9. Program secondary settings into infusion device if used.
10. open clamp on secondary bag tubing.
11. check that primary infusion resumes as its 'set' rate when secondary volume has been infused.
12. When secondary bag is empty, readjust rate of administration to desired flow
13. to add a new secondary bag, assure that medication is the same as previously administered since some drug remains in the tubing. A different medication requires its own secondary tubing.
14. Remove old secondary bag, and spike new 'piggyback' bag.
15. Lower partial-fill bag below injection port of primary IV
16. Open clamp set on secondary tubing, and allow solution from primary IV set to enter tubing, backfilling the tubing to drip chamber."

sorry so long. should answer all your questions. we had to get this book
Smith, S. Duell, D., Martin, B. (2004). Clinical Nursing Skills Basic to Advanced Skills 6th ed. Pearson Education, NJ.
If your instructor has a reputation for being nasty about skills, I totally recommend the book. Goes over how to do pretty much everything.