Thanks in advance.

You have to remove the fore arm from the gun.
Inside the front of the fore arm is a smooth nut with two slots.
This nut has to be unscrewed.

There are a number of options:
Buy a shotgun fore end wrench from Brownell's.  This is rather expensive for a one-time job.
See if a local gun shop has a wrench and will do it for you or lend/rent you the wrench.
Make a simple "spanner" from a piece of flat 1/8" steel that will fit, and use a Vise-Grip to hold it and turn the nut.

I haven't had time to test this, so NO guarantees:  A man told me that he used the bolt's operating slide as a spanner on his gun.  I'd be careful if I tried this so as not to damage the slide.

Be VERY wary about advice to use a pair of needle-nose pliers to unscrew the nut.  One slip and you can seriously damage the thin fore arm tube.

Another man says he used a large wood boring bit that fits the notches.

U will need to take the pump action apart from the gun. Take your finger inside the action from the bottom and push in the tab on the left. Take the barrel of first.

I recommend a purpose made wrench. If your forend is factory tightened or is old with accumulated dirt, crud, etc in the threads, even with a wrench, it can be a bear of a job.

Remington does an excellent job of making original owners manuals available for download.

Remington

You'll find the foregrip replacement is a snap.

If you have a mag ext. you have to remove the clamp and ext. first, then remove the barrel, put your thumb up in the receiver and push the left shell latch in, and slide the forend off of the mag tube (see pages 12-13). Now you can either buy (Remington recommends) the tool from Brownell's for I think $49.99, I think Galati sells a B square wrench for a little cheaper, use the 1-1/4" wood bore bit Bigger_Hammer recommends for under $10.00, or I'm sure dfariswheel's trick works, he always has good info. I did this last month and had a rough time, but these guy's got me through it.

Surefire light forarms come witha stamped thick sheet metal tool that works perfect.

Thanks everyone!!

try this link...  Pretty good run down on what yer lookin fer...

www.shotgunworld.com/bbs/viewtopic.php?t=18593

Get the dedicated 870 spanner wrench. It's worth it especially if you own a number of different makes of shotguns.
It works on Mossbergs, Remingtons and Ithacas.
It's a tad tight on the Mossbergs and Ithacas but it can be made to work A-OK with a little coaxing.

And hell, it's like $20.
What gun guy refrains from buying a useful, purpose built firearms tool for $20??