Here are the tools you will need:
Some Paper Towels or soft cloth
Some High Temp grease, like Mobil 1
Sockets/ratchet for removal; I needed 6mm and 5/16"

To get started you need to get the Diverter Valve (DV) out of the car. I've already swapped out my stock intake box for dual cone intakes, and I found it easier to remove them to get the DVs out. I removed the filters and couplers together, and for that I needed a 6mm socket for the stock hose clamps.
Once you get the filters or stock airbox out of the way you can easily get to the DVs. You'll need to loosen the hose clamps to get them off; mine needed a 5/16" socket. Once you've loosened the clamp just pull the DV out.
Now take one of the DVs to a clean surface to work on.
Start off by unscrewing the top and getting the spring, piston, and any spacers out.
Go ahead and wipe everything down with a clean cloth and then take the o-rings out too.
Once you've got the o-rings out you'll want to wipe them down and check them for damage (Forge sells a service kit with new o-rings if yours are damaged.) And then clean the o-ring recesses in the DV.
Once you've got that done, give the o-rings a light coating of grease, and then get them back in their grooves.
Next, make sure the piston is cleaned up and give it a light coat of grease too.
I'm using general high temp grease from the parts store.
Now set the piston upside down on a clean flat surface and set the DV down over it and gently slide the DV body down over the piston.
It should look something like this when you're done. Now get your spring out, as well as any spacers you're using.
Put the spacer(s) in first, if you're using any, followed by the spring.
Now just make sure you greased the top o-ring a little bit and then put the top on and screw it down hand-tight. Do your other one and then get them both back in the car. Installation is the reverse of removal.