First, I think that's rather rude of your yarn shop to refuse to help you just because you didn't buy the pattern there! After all, you DID buy your yarn from them!
Now to your question! Here's what you need to do:
Once you work that "Next Row", you will have 2 sections -- let's call them A and B -- containing 13 sts each and they will each have their own yarn with which to work them. From this point on, you will work each section using ONLY the yarn attached to that section.
Following Row: Work all 13 sts in Section A, switch to Section B and BO the first 3 sts (this is the neck edge), then work to the end. Sect A=13 sts, Sect B=10 sts.
Next Row: Work all 10 sts in Sect B (last section worked). Switch to Sect A and BO first 3 sts, then work to the end. Sect A=10 sts, Sect B=10 sts.
Next Row: Work all 10 sts in Sect A (last section worked). Switch to Sect B and BO first 2 sts and work to the end. Sect A=10 sts, Sect B=8 sts.
Next Row: Work 8 sts in Sect B. Switch to Sect A and BO first 2 sts, then work to the end. Both sections should now have 8 sts which you will put on holders for the shoulders.
I hope that helps. Good Luck!