Measure your bunk bed to determine whether it is standard sized. A standard twin-sized mattress measures 39 inches by 75 inches. You'll want some draping from your comforter, but only a few inches on each side.
Collect the appropriate materials. Buy fabric to size from the fabric store, or piece together your own from leftover fabric at home. You'll need a top piece of fabric, a bottom or backing piece, and batting for the inside. The top of the comforter and the backing will need to measure 47 inches x 82 inches. Standard twin-sized batting measures 72 inches x 90 inches, so prepare to trim.
Once you have the fabric ready to be sewn together it's time to piece the comforter together with the batting. Clear an area on the floor where you can spread everything out.
Trim the batting down to match the size of the top and the backing. Get your pins ready. Lay the batting down first. Spread it out so there are no folds or lumps.
On top of the batting, spread out the backing, right side up. Seams in the material should be facing the batting. Smooth everything out again so there are no folds or lumps.
Spread your bunk bed comforter top piece over the backing. This time, you'll want the "wrong" side (the side with the seams) facing out. Don't be concerned that your initial sewing will be done with the comforter top piece inside out. You'll pull it right-side out later in the process.
Pin the corners together through all three layers to ensure that all the sides remain stable.
Sew three of the edges together with a half-inch seam to form a large pocket. You will sew the two longer sides and one end, which will be the bottom of your bunk bed comforter. Leave the top end open with a half-inch gap.
Reach between the top and the backing and turn the comforter right-side out. Make sure the corners are pointed out. Lay the comforter out on the floor and smooth out any lumps or folds. Pin the comforter at equal intervals vertically through the middle to hold the batting in place while you sew the top closed.
Fold the raw edges of the top inward. Sew the top shut.
Sew the bunk bed comforter down the middle where you've pinned it. Make sure that no folds or lumps get caught in the fabric and that the thread goes through all three layers. This will secure the batting inside the comforter and ensure that your quilt can remain intact through future laundry cycles without ripping the batting free.
Sew the comforter at regular intervals, such as every 3, 5, 6 inches or more, casting your stitches in designs of squares, ovals, circles or diamonds.
Once the sewing is complete, run your bunk bed comforter through the washer and dryer to secure the stitches. Finally, spread it out across the bed and enjoy.