I like to buy the 6 x 9 drop cloth as it doesn't have seams.
I have a queen size bed. I don't have specific measurements so make this to fit whatever size bed you have. READ ALL INSTRUCTIONS BEFORE STARTING! There are options you can choose.
Get rid of the mattress and measure the dimensions of the box spring. Get two measurements, length and width, cut your base add an inch on three sides for the seam allowance. OR you could do what I did and lay the drop cloth on the box spring and with a marking pen mark where the edge (seam) to the skirt will be. I added an inch all the way around for seam allowance and cut. For the top of the bed where you will not have a skirt I used the drop cloth original hem. Why not use the factory edge, nice clean finish. And I won't have to hem that side.
Now, decide how long you want the skirt itself. Keeping this in mind and your length and width measurements. Cut two for the length and one for the width. Add enough on the bottom for a hem and enough on the top (1 inch) for the seam allowance. OR you can add pleats to your skirt like I did. Check out how I made pleats for the Slipcover. For the bed skirt I wanted a pleat half way down the length at the corners and in the middle of the end section. Using the measurements from the Slipcover project I cut appropriate lengths. Hem each section before pleating it. Makes the layout easier. Pin the skirt to the base and sew. Add each section as you go. Below you can see several views of the skirt on backwards.
Ellie wanted to get in the photo.
I kind of wish I had put an additional pleat at each corner but overall I'm really happy with it. I've washed it already and it still looks awesome. There are endless uses for these drop clothes. I hope to make dining room curtains next! Check back in to see.