Take a look at the pic linked below:
http://www.performance-pcs.com...ges/RubberScrew_02.jpg
Notice that on either side of the large "washer" in the center is a gap...but the gaps are of different sizes.
The larger gap is to fit the fan frame, the smaller gap is to fit the case.....and the rubber washer remains between the fan and case panel.
First, place the tit with the larger gap into the fan's frame hole and pull, work, wiggle it into the hole until it "snaps" into place. As mentioned before, a little lubrication of water or other wet substance can help.
When you've finished with all four corners of the fan, it should look like this:
http://www.performance-pcs.com...ges/RubberScrew_03.jpg
Now, you do the same thing with the case......place the fan to the mounting holes in your case with the exposed tits poking through the fan mounting holes in the case panel. Again, stretch, wiggle, work the tits through the holes until they, too, snap into place...and again, some wet lubrication, like water, saliva, etc., can help a bit.
I have found that in the second step, sometimes it helps to restrain/hang onto the tits that are already mounted in the fan's frame.....this sometimes helps from pulling the rubber screws out of the fan's frame when you are trying to get them through the case panel.
When you get all four in, you're done.
Of course, and I'm not trying to be condescending or anything, but you do realize this type of rubber mounting screw only works on open corner, or ribless, fans. Closed corner, or ribbed, fans need the long rubber screws like I had linked in my original post.