A couple of steps for cleaning an 870.
1) make sure it is unloaded.
2) unscrew foreend cap.
3) Press slide release button in front of the trigger and slide the foreend back.
4) remove the barrel.
5) clean barrel.
6) Take a small nail and tap the pins out of the receiver.
7) Gently pull out the trigger group.
8) Clean trigger group.
9) Turn the receiver upside down.
10) identify the shell latches. There is one on each side of the receiver. They are the springy pieces of metal.
11) you need three hands for this next step. But can do it with two. Press the two shell latches in and gently pull the foreend forward until it is loose.
12) now you see a loose foreend with attached slide rails and two other pieces.
13) Degrease and lightly oil all this.
14) clean the grease out of the receiver.
15) put the two pieces that came out with the foreend back onto the forend. It fits in the grooves on the foreend.
16) Depress the shell latches and slide the foreend back in with the two pieces assembled on the slide rails of the forend.
17) Put trigger and pins back in.