I've got one of those bedside cribs for Little Pickle. The mattress is a really unusual size so we're in need of a custom fitted sheet.
Start by measuring the crib mattress - length, width and depth. Mine is 22" x 36" x 2". So my fabric needs to measure 22" (width) +2" (depth) +2" (depth) +2" (hem) +2" (hem) = 30" x 44".
I've folded my fabric in half to make it easier to work with. I've cut a 4" square from each corner.
Each side of the square is brought together and sewn.
I've also overlocked all the edges. You could hem them if you don't have an overlocker.
Them I've pressed the hems over by about an inch.
Next I've pinned a length of elastic along the shortest edge of the sheet.
I've secured the ends of the elastic as I've sewn the hem in place.
The fabric is a brushed cotton jersey from my stash.