I would start with getting a book first. Simply Slipcovers
is a pretty good one to start with. Or you could look around places like Walmart or JoAnn's for some patterns. I know McCall's has a pattern and I think Butterick does, too.
Here's what HGTV says about creating a custom slipcover:
- First, create the pattern pieces by tracing the outline of all planes of the chair (or other furnishing) onto muslin or other lightweight fabric.
- Next, cut out the patterns and pin them onto the upholstery fabric. Then, carefully cut out each piece, allowing an extra half-inch around the edges for seams
- Finally, sew the pieces together to complete the slipcover.
I hope this helps a little bit!