Measure the longest brush to be stored. Double that measurement and add 6 inches (4 if the brushes are small) for overlap and seams to find the length. Count the brushes and allow 1 inch of width per brush (2 inches for very thick handles) and add 3 inches to the total for the width.
Cut a piece of fabric the length and width determined by Step 1. Sturdy fabrics like canvas work well for a project like this, but if you want to use a printed or other "fashion" fabric and not be looking at a drab underside, cut two pieces and sew them together, either right sides facing and turned out or leaving a narrow hem to be enclosed later.
Mark the center of the long side of the fabric and fold the bottom up to that mark, turning the rough edge under twice and pinning into place. There's no real need to sew that hem separately, since the amount of stitching that will cross it to make the pockets will hold it in place. Press the folds with an iron.
Starting 1.5 inches from the left side, measure the 1-inch (or larger) pockets for each brush and pin along the stitching line for each. Even if you choose to mark your stitching lines with chalk or disappearing-ink markers, still pin the fabrics periodically to keep them from shifting as you sew.
Sew along your marks, using a straight stitch or a tight zigzag between the bottom edge of your caddy and the end of the overlapping section. Cut the thread between each channel close to the fabric.
Hem the exposed edges of your caddy with a double fold hem or enclose them in bias tape or wide ribbon, folded over the edges and stitched in place.
Attach a long length of ribbon to the right edge of the caddy, just above where the pocket edges end, by sewing through the middle of the ribbon or cord.
Insert your brushes, fold the top edge of the caddy down to cover the brushes and overlap the pockets a bit, roll it up from the left to right and tie the ribbon or cord to keep it closed.