First thing is, you need to remove the ft hub and washers and stuff;
Welcome to guzzle's 4x4 Needle Bearing Maintenance Web Page
(lube those needles while you're in there)
Next is the Knuckle Seal and it's around $60 plus, you're gonna need to make a tool to install it because, chances are, you're going to have to destroy it getting it out;
Ford Super Duty Knuckle Seal Tool - AKA OTC 6695 | Superdutypsd.com
Then, if you have vacuum hubs, you should replace the Yellow O-Ring on the hubs. If you have manual hubs, and if you're lucky you might even be able to get the Knuckle Seal out in reasonable shape so you can reuse it and certainly just reuse that Yellow one too (you don't need a good seal with Manual Hubs).
Now, to go farther into things you need to pull the axle. It just pulls out (can be kinda stuck but, give it a good "pry"). Here's a vid of a Ball Joint change that's pretty good. Just ignore removing any steering components you don't have to do that for the U-Joint.
Finally, once you get the axle "in your hand", you'll need to take the joint apart and Install the new one;
That should do it!