You can customise this to your ideal size, style etc.
First collect your equipment:
-Some fabric (I used some scrap material from an old chambray shirt, so it's pretty lightweight, you could easily use something more sturdy if you want)
-Pins (not really necessary)
-Dressmakers measuring tape
I also just want to say I do not think this the best way to make this but this is the way always do it and works fine for me so I hope it works for you too!
Step 1: Measure
Step 2: Sew
-Leave one side open
-Turn it inside out
Step 3: Sew Final Side
This bit is a bit hard.
-Fold the edges of the open side inside
-Pin it here if you need
-Sew along this edge
Step 4: Make the loop
-Fold in the sides and pin if you need to
-sew along these edges
-Fold over this strip and pin if you need
-Sew across, about 1cm from the bottom
Step 5: Assemble
-First I fold the rectangle in half then back on itself (I hope the picture explains it better than me)
-This gives it that cute bow look, I think anyway
-Then just push through the loop and adjust it so it looks good