With WRONG sides together, fold the bottom short edge of your linen rectangle up to meet the top short edge. From the opening, mark down 2.5" on the left and right sides. Stitch 1/4" seams on each side of the bag from bottom to top--stop sewing when you reach the line you have marked. The unsewn edges will be the casing later.
Trim the corners of the seam allowance carefully at an angle (this makes nice sharp edges to turn out) and turn bag out. Press. Now the RIGHT sides will be together. Re-mark the 2.5" marking you made earlier.
Stitch a 3/8" seam on each side of the bag from bottom to top--again, stop sewing when you reach that 2.5" mark. This 3/8" seam will completely enclose the earlier seam.
Turn the bag right sides out. Press.
You now have four side edges that are raw and need to be hemmed. Fold each of these edges towards the wrong side of the bag, roughly 1/4" over and then a 1/4" again.
Hem each of the raw SIDE seams this way. Leave the top edges until the next step.
Here's what it'll look like:
On the top open edges of the bag, starting with one side, fold down 1/4". Press. Fold this edge down 1/4" again and press. To be honest, I just eyeball this. If you make it too thick, your bias tape might get a little squished when you add it in, so try to be accurate, but it's really not the end of the world. Repeat on the other edge.
With the top edge folded over as pressed (1/4" and 1/4" again) stitch the seam. Repeat on the other edge. NOTE: I realized that half the time I make these, I don't even bother with this seam, instead I just press my fabric well and go right on to the next seam. Feel free to do it whichever way you like. Fold the first edge down in half (it will now line up right where that original 2.5" mark was from the beginning) and stitch. Repeat on the other side.
Use a large safety pin to thread bias tape through both sides of your bag. When you arrive at the entrance point, measure about 2.5" extra on each side and trim. Use this excess to tie a tight knot.
Now, starting at the opposite side of your bag, thread the bias tape through again and repeat.
Trim any excess threads and you are done!