I simply measured both sides of the pad.
I used an old twin sheet and used those same measurements. It was 23 by 35. I made sure to use the finished end of the sheet for the top (see below). That way I would already have a finished edge to sew the snaps on.
I simply sewed up the three sides - leaving the already finished end at the top. Serging makes the ends much more finished.
Simply slide the sheet onto the mattress to make sure it fits. If it's a little too loose somewhere just take in that side to make it fit nice and snug.
The opened end needs to have snaps sewn onto it. I haven't quite gotten around to this yet. . . It's my project while watching "my show" tonight.
Doesn't it look so much more comfy?