Here is how I install the universal mud flaps, which I have done on a lot of cars over the years.
With the car sitting on level ground, place a 2 x 4 behind the front wheel, below the rearmost part of the inner fender (near the door). Place the mudflap against the inner fender, slide it down until it rests flat on the 2 x 4, move it out from the fender about 1" or so in order to add some protection as the wheels are steering and throwing up debris. Take a marker or tape, and mark a few spots on the fender (I use painter's tape on the paint) to locate the mud flap vertically and cross-car. Jack up the car and remove the front wheel. Take #8 self drilling sheet metal screws with a flange head or washer and screw them into the upper corner of the mud flap. Line up the marks on the mud flap to the ones on the fender so that it will sit in the spot you intended while the car was sitting on all 4 wheels. Run the self drilling screw into the inner fender. Hold the flap from rotating, double check the alignment with the marks, and add another screw into the inner fender near the metal fender in the upper corner. The flap is now secured at the top and will hang down. Use another screw to go into the lower side skirt, and then another near the center of the car. If needed, add another in the middle of the flap. Repeat on the opposite side. It takes longer to write this than it does to do the fronts. I did it to my wife's Escape in just a few minutes.
The rears are not as easy. You still start by marking the flaps and use a 2 x 4 to get the height and parallelism correct. My 2011 Fiesta with the body kit is really easy since it has a large flat surface with screws that work perfect for attaching the mud flap to. The ST doesn't have that, so you need to build a L bracket using some stuff you find from a hardware store or Home Depot. I use a #8 or #10 bolt, nut, and washer to secure the bottom of the L to the bottom of the rear bumper cover in front of the felt liner. Make the vertical part of the L flush with the arc of the wheel well. Line up the flap with the marks to get it in the right place, BTW, the wheel is off and the car is jacked up at this point. Shine a flash light thru the hole in the vertical part of the L bracket (you may have to drill the hole if the bracket came without one). Mark the hole on the flap where it shows up using the flash light. Drill a hole in the flap, run a flat head screw thru the hole and a nut and washer on the back of the L bracket. You don't have a lot of room to work, but it can be done. With the marks lined up, add another self tapping screw in the corner near the bumper cover, about where the quarter panel and inner fender meet. There is a metal tab that holds the inner fender to the frame rail. Drill a small hole into the tab and run a self tapping screw into the tab to secure the flap. I run a zip tie from the flap to the chassis tie-down bracket to keep it from rubbing on the wheel. The rears take a little time, but aren't too bad.
I should really make a video showing how this is done since I get asked a lot about it. I have done 4 Focuses, 2 Fiestas, a Focus ST, an Escape, SVT Contour, and an Escort GT. I might have some pictures on my phone I can post to help show this for everyone.