If you have an existing Bermuda bag cover, you can try tracing the pattern and adding seams, but if this isn't your forte, go the easy route. Download or purchase a pattern that fits your bag here or download it as a PDF here.  Be careful to choose a pattern that aligns with the style of your original bag. If you're glowing with excitement after the recent popular gift with purchase, this pattern will fit perfectly! ;)

Clip edges where noted for easy seams around curved edges. Don't forget to cut notches for your gusset line, center top, and center bottom.

Adding piping gives the perfect, finished edge to your bag, but it's optional.  If you are new to sewing (or don't know how to work with piping) you can opt out!  I worked through without a piping foot the first time with decent results, but if you're willing to invest a bit, the sewing attachment makes the process so much easier.

Using the pattern mentioned above, choose which pattern will be your primary pattern, and which will be lining. Don't worry--the pattern above is reversible. 

For your primary pattern, pin piping around edges, leaving a bit of overlap at the center bottom of bag.

The inside lining will "borrow" the shared edge of the top, so you only need piping on the bottom seams. This will make a less bulky seam at the sides. Clip and baste 1/4" from edges.

Clip the edges (not shown) as indicated in the pattern to ease together seams along the curves.  

Start with the primary cover and begin pinning (or clipping) right sides of gusset from the center seam, moving along curves to the fold line of the right side of the cover.

Clip seams inside bag as needed and flip, pushing lining inside of outer, primary bag.  Close seam opening by topstitching, or hand stitching.