How do I disassemble my 870 Security?  I tried to disassemble it by following the steps in the manual.  I managed to remove the mag extension but the magazine cap won`t budge?  What gives?

Also, I will be installing a side saddle, but the closer end of the forearm overlaps the receiver leaving no room for a side saddle.  How should I shorten the forearm?

Often the fore end cap is screwed on too tight on Remington guns, they don't need to be torked down that tight.

To get it off, wrap heavy cloth or a piece of thin leather over the jaws of a pair of large pliers, grap the cap, and unscrew it.  Be careful not to scratch it.
When you replace it, just screw it down finger tight.

For the fore end, you will either need to buy a short Police-type fore end, or simply cut the one you have off with a fine-tooth saw, sand the edges smooth and rounded slightly.

I managed to unscrew the cap, but I`m still having problems with the fore end. How do I remove the action bars from the fore end?

You need to remove the whole forend from the receiver.  I believe on the 870 there are two pins on the receiver that need to be pushed out, then the trigger group can be removed.  Once the trigger group is out the forend can be removed.  Depress the shell retention lever and the forend should slide out with the bolt.  Once the forend is out you need to use a spanning wrench to remove the nut on the end of the forend assembly.

I am a little rusty on 870 disassembly, so if anyone sees mistakes, please correct me.

Joe's on about the disassembly.

If you're careful, you don't really need to go to all the trouble of removing the wood fore end from the action slide and bar unit.

To protect the metal, slip a piece of sheet metal, or even a thick cloth under the wood, between the wood and the metal bars.  This will prevent hitting the metal when you saw the wood off.  Simply make small cuts, working around the rear of the fore end until it breaks through and can be removed.
Then, use a sanding block and some medium sandpaper to smooth the cut end, apply a couple of coats of your favorite finish, and it's done.

This will eliminate having to buy a fore end wrench, for a one-time job.

Fieldstripping:
1) Remove mag tube nut (and mag spring)
2) Remove barrel
3) remove two retainer pins with a dull punch
4) remove trigger assembly

Here is the only tricky part:

5) Inside the rcvr, squeese the two arms that retain and release each shell from the mag tube. Simultainiously pull the forearm out.
The bolt should come with it.

6) Remove the forearm buy removing the slotted nut.