The shrug pattern I am working on starts out in circular knitting for the first sleeve. Then is says now knit flat for 84 rows. How do I switch from it being in circular knitting to start to be flat knitting??
When you're done knitting in the round, just turn the work and start on the back side. It'll probably feel awkward for a few rows. If if's easier you can use another (same size) needle to purl across the first time and then alternate needles for a few rows. It will no longer be in the round and will be obvious after a few rows of front and back knitting and will be easier to use just one needle.
When asking questions ALWAYS post the name and a link for the pattern if you have it.