Taking the Measurements
A standard crib mattress is 52″ by 27½″ by 6″. If you are making a set for a portable crib or other crib that's not of standard size, measure the mattress's width, length, and height carefully. The bumper pads and sheet should fit exactly. If they are loose, they can become hazardous if the baby gets entangled in them. If they are too tight, they might tear, becoming just as hazardous.
To make a fitted sheet, you will need one piece of fabric the width of your mattress, plus twice the depth, plus another 4″ for the hem, which will also serve as the elastic casing. For a standard mattress, that comes to 43½″, or approximately the width of most fabrics. The length of this piece will be the length of the mattress, plus the same as was added to the width. This comes to 68″ for a standard crib. Since you will lose 1″ or 2″ at each end of your new fabric when you straighten the edges, buy 2 yards of fabric for your sheet if it's a standard crib mattress.
For a 42″ by 34″ comforter, you will need 1 yard each of two coordinating fabrics. If you want to add a 2½″ homemade ruffle around the edge of your comforter, you'll need an additional 1¼ yards.
Crib set: Coordinating sheet, bumper pad, and comforter.
To make the bumper pads for a standard bed, you will need 2¾ yards of fabric. In addition, you need an optional 1½ yards for a ruffle and either ½ yard for ties or 4⅔ yards of woven ribbon that's ¾″ wide. These ties will be lined with self-gripping fasteners, usually called Velcro. If you want to tie your bumper pads in place with bows, you will need about half again as much fabric or ribbons.
Unless your crib is a great deal smaller than standard, you will still need to get the 2¾ yards for the bumper pads. You should be able to cut your ties from the leftover fabric and get by with 1¼ yards for the ruffle.
As fun as it might sound, resist the temptation to make a matching pillow for the crib. Pillows are a suffocation hazard and should never be placed in an infant's crib. An older child might also use it as a step and fall over the rail.