I could have easily made a one piece bed skirt that would have been long enough to touch the floor, but as we lower the mattress, this would have involved altering the bed skirt for each new level.
Instead I decided to go with 4 seperate panels that could easily be adjusted for each stage. I started by measuring the length and height of each side.
I cut my navy material (a bed sheet that matched our accent fabric) so I would have 2 side panels and a front and back panel. I made the front and back panels a little bit wider than the crib so there would be a little overlap to tuck under the side panels and we wouldn't have gaps at each corner. I also added a 6" panel of striped fabric to the bottom to give it a little more detail.
Once I finished each panel they were ready to be attached to the spring base. Since the mattress will do a good job of holding them in place I just used sticky velcro to tack it down along the frame. This way I will be able to remove and reuse the velcro as we lower the mattress and move the bed skirt up.
I started with the widest front and back panels so I could tuck the extra fabric around the corner. Ignore the missing floor boards (another DIY project to replace those is in the works!)