If you want to weave fabric wider than your loom and you have extra shafts available, you can weave your project in two layers connected at one side. In effect, you’re weaving it folded in half lengthwise, and once you take it off the loom you can open it like a book.
The actual weaving isn’t trivial – you need to watch your draw-in on the folded edge, you need to make sure you don’t inadvertently stitch the layers together by crossing the weft with a thread from the wrong layer of warp, etc. But before you can worry about any of that, you need to know how to change the draft.
Software can do this in a heartbeat, but not everyone has software, or has it handy when they need it. Working out at least a few drafts by hand will also help you better understand exactly how weaving a cloth folded in half works.
This tutorial takes you step by step through the process of folding a draft in half lengthwise, using this draft as an example:

