They are pretty straight forward, I think.
I have not had an 880 apart, but it should be similar to the Powerline 822 in many ways.
In the case of the 822, the buttstock must be removed with a 13 mm socket and extention up through the deep hole that hides beneath the buttplate.
Then the forend halves are removed. The cocking lever is pulled back, and the two halves of the receiver are separated after removing the screws in the sides. The barrel shroud (outer sheetmetal tube) can be slid forward to remove it, and then also the pump cover slides off. There will be 2 pins that may fall out but that shouldn't be a problem. The plunger should come out of the pump with or without the pumping lever or "foot" still pinned at the hinge point, and the outer cylinder slides forward to come off. It is a tight, O-ring fit, but should come off and expose the base of the air valve.
Note that I may have this a little mixed up in my memory, and in any case, it is not the model 880, but a similar type. Still, it isn't too hard to get it opened up as long as you pay attention to how it does so and don't lose anything.