Here you go!
Some people just use cloth napkins. They never fit right anyway. Very simple fix.
You're gonna make 2 template pieces. That you'll use cut the fabrics sew.
Grab some sticky tape, scissors, large pieces of paper, news papers will work fine too.
To figure the template for the front of the arm rest #1, wrap it, and tape to keep in place. Then drape the second piece of paper mimic the top of the arm rest #2. Fold or cut until get neat edge on #2. Then mark on #1, where the 2 paper template meets. Cut along mark on template #1.
Now you have templates, place the fabric upside down, and put your templates on the fabric, and mark it either with chalk or marker. Now have one arm rest marked out, next you'll wanna flip the templates to create a mirrored arm rest cover, mark them out. Once you have 4 pieces of the arm rest marked out, cut the fabric leaving an extra inch on all sides.
Next you'll construct the arm rests inside out, by pining them along the pen/chalk mark every half inch, and this is the important part, make sure aline perfectly because that's pretty much how they'll turn out.
Now sew along the marked line. I using sewing machine using straight stitch.
You can also fold and pin outter edges and sew them up for a neat look.
Some folks will embed a small stiff plastic rod where the 2 fabrics join.
By creating your own templates, you can pretty much create anything want. Good luck.