First thing I'd like to say is they are not like the ones in the photo. In the photo they look like they hook on under the door frame, they do not as they use bolts into the roof the same as the ford ones.
Secondly when you take the plastic cover off the roof of your Focus, don't panic!!!
I say this because I did as there was no holes to mount the bars into via the allen key bolts. They are there though under a layer of painted mastic. I just cut a square out from the centre of the mastic and used a flat headed screwdriver to scrape the mastic out, under which there was a small screw that comes out that is protecting the holes.
After that Just folow the instructions on the box and ta da, save yourself a fortune as the Ford ones are really expensive by comparison and do the same job. All in all well pleased, would highly recomend :-)