Use Full Length Poles For Bay Windows
When creating a bay window treatment, make sure to get full length poles. Don't ask that your poles be cut to size beforehand, you're going to need to have full sections to work with. The actual size pole you're going to need will be different than your physical wall or window width because the return size of your bracket will be projecting your drapery out, altering the actual width you'll need. It's fairly difficult to determine the deduction beforehand and any irregularity in your wall can also impact the final size.
If you're tackling a three window bay, for example, the first step is to install your support brackets. You'll need to use at least two brackets for each window in the bay. Swivel sockets don't provide support or stability. They just continue the look of the pole. They are meant to look pretty, not to provide strength. When hanging the brackets, make sure they are level across all sections of your bay window.
Dry Fit Each Curtain Rod First
Once your brackets are up, you can dry fit your first poles. Place your poles on the brackets and carefully mark each one where it's going to need to be trimmed. Remove the first pole and trim one side down, attach the swivel socket and verify your measurement. Once you're satisfied, you can mark the other side and trim it down too. Continue the process for each window in the bay. And, depending on the size of each section, you may need to pre-trim larger poles before you dry fit them. But, take as little off as you can get by with, you don't want to run out of pole.
Corner Windows Are Easier
Corners are a lot easier because you won't be dealing with multiple angles. You can follow the same process and start by hanging your brackets. Again, at least two brackets for each pole section.
Limitations When Using Rings
Also, you'll need to plan to treat each section independently if you're hanging a curtain with rings. Your rings aren't going to be able to move past the swivel sockets, even if you're using bypass brackets and bypass rings. For operable drapes that move around the corners of bay windows, you'll have to consider a custom bent heavy-duty curtain rod instead.