Making Fitted Corners
Let's begin with the fitted sheet. From the trimmed edge of your fabric, measure 67″. Pull a thread, and cut across the width of your fabric. Measure from selvage to selvage to be sure your width will still be 43½″ after the selvages are trimmed away. Trim them away if you can, and cut down the length of the fabric to make a piece 67″ by 43½″.
This is one project in which it won't matter if you have to leave a selvage along one edge. The reason you should normally cut it away is because it will sometimes shrink. In this case, that won't really matter. You will still need to hem the edge so you have a casing for the elastic.
If your bed is not a standard size, cut the fabric to the calculated dimensions.
Fitted corners: Mark the stitching line.
Fold, stitch, and turn.
To make a fitted corner, mark off lines that define an 8″ by 8″ square at the corner of the fabric (as shown in FIGURE 9-2). If your mattress is other than 6″ deep, mark off a square equal to the depth of your mattress plus 2″. This should equal half the amount you added to the width and length of the mattress.
Fold a corner on the diagonal, match the marked lines, and stitch along them. Trim away the corner ½″ from the stitching (see FIGURE 9-3). Zigzag-stitch along the raw edge of the seam allowance and press to one side. Repeat with the other three corners.
You shouldn't put plastic sheets on an infant's bed because of the smothering hazard. Most crib mattresses are waterproof, anyway, but if yours isn't, protect your baby from the plastic with a quilted mattress pad over the sheet. You can find already quilted fabric and can make the pad just like a fitted sheet.