Numerous guides exist online for making bike frame bags with good tips on sizing, fabric, and construction. There are many excellent custom bag makers that can be used for design inspiration. Basic steps are as follows:
1. Make a template inside the front triangle using cardboard or bristol board. Make sure to mark any cable stops or water bottle bolts that you might want to avoid or use. This template (plus seam allowance) will be used to cut out the side panels but it's also useful for planning the location of zippers, attachment points, and internal pockets or dividers.
2. Cut your fabric: two side panels, one strip to run along the "spine" of the bag where it contacts the frame, and any storm flaps, pockets, dividers, stiffener sleeves, etc... I used a seam allowance of 3/8" and a finished bag width of 2 1/4". Zippers are best sized as long as possible to maximize access.
3. Sew zippers, pockets, attachment points into the side panels and any sleeves for stiffeners into the long spine strip. I found it helpful to tack attachment points like grosgrain and velcro onto the side panels before sewing the bag together.
4. Sew the whole bag inside out. Start by sewing one side panel onto the long spine strip followed by the other side panel. A straight stitch worked fine for me. Flip the bag inside out through the zipper opening, insert any stiffeners, attach to your bike and enjoy.