I guess that's what you call it? That's what I'm calling it anyway.
So here it is - it's really simple. I think you should measure your stroller handle first just to make sure you don't need to make it shorter and/or wider (or skinnier). I made this one based on a Bob jogging stroller and it is a little loose (you just don't want to make it too tight and then you can't get it on!)
1/2 yard of fabric
8.5" x 26.5" piece of batting (you could also use fleece)
6 sets of snaps (I use these - size 16)
Cut two pieces of fabric measuring 8.5" x 27" long (again - check your stroller first and determine your measurements from there).
Lay your fabrics right sides together and lay batting on top of fabrics.
When you lay the batting on top of the fabrics, your fabric will be 1/2" longer at the top edge - this is correct. You don't want to have too many layers when you fold your fabric in to close it.
Sew along three edges with a 1/2" seam, leaving top open.
Turn right sides out and iron all edges flat.
Fold fabric in at the open edge and iron. Pin opening closed and then sew around all 4 edges using a 1/4" seam.
Now you'll have one long piece.
You will place one snap at each end and then place snaps at 5" intervals across the whole cover.
Make sure you place them so that when the handle bar cover closes, they snap together (it's easy to put them wrong, not that I would know anything about that - ha ha!).
You're done - I told you it was easy! Now go get your stroller and snap on the new cover!