Hi BobMc98. I'm glad you came to the community.  Let me walk you through the steps on how to do this:

1. From the Vendors menu, pick Vendor Center.
2. Select the vendor's name.
3. In the Vendor Information section, find the Bill under the Transactions tab and double-click it.
4. Choose the Select PO icon.
5. Check all the purchase orders that need to be included on the bill.
6. Click OK, then Save & Close.

Check out the article on Accounts Payable workflows in QuickBooks Desktop for some in-depth information about how everything links together. 

Let me know if this helps, and if you run across anything else be sure to check out the QuickBooks Learn and Support page for more great how-tos and articles. Don't hesitate to leave a reply below, I'm always happy to further assist.