Stuff sacks are nice DIY project to get you started, as they are relatively simple with straight lines. The square bottom stuff sack helps give them that professional look, and adds some structure and strength.
This sack that I made was for my hammock hex tarp with a ridgeline of 3400mm, tapering down to 1600mm at the sides. It is made from lightweight Xenon Sil available here. Other materials used are a tiny cordlock and 1.75mm Lash-it cord.
Step 1: Cut the fabric - mine was 400m x 400mm.
Step 2: Sew a rolled hem down one side just inside the outer side.
Step 3: Sew a rolled hem down the opposing side
Step 4: Sew a rolled hem along the bottom
Step 5: Sew a wider rolled hem at the top. This will be the channel you run your cord through to cinch the sack closed.
Step 6: Fold over the sack bring together the sides you sewed in Step 2 and 3.
Step 7: Sew a line of stitching inside the stitches you sewed in Step 2 and 3, continuing down and sewing along the bottom and again inside the stitches of the rolled hem. Note: At the start of your stitches make sure you backstitch a couple of times not only to lock the stitch but to add a little strength to the seam at the top of the sack.
Step 8: Measure the width of you sack, mine was 160mm, and divide by 4, this gave me 40mm.
Step 9: Pull the sides perpendicular to the bottom seam so that you form a square of fabric, or diamond shape depending on how you want to look at it.
Step 10: Measure along the bottom seam 40mm (or whatever your measurement is from step 8, mark, and repeat on the other side of the bottom seam.
Step 11: From the marks in Step 10 measure 40mm perpendicular to the bottom seam on either side and draw a line. Repeat on the other side.
Step 12: Sew a single line of stitches along the lines you marked in Step 11.
Step 13: On the outside of the stitches sewn in Step 12 cut the corners off.
Step 14: Turn the bag inside out and run some cord through the top channel and then install a small cord lock to enable you to cinch the sack closed and tie an overhand knot.
Step 15: Insert tarp and smile.