- look to your control panel, find the section labelled "modulators"
- select the number of the modulation slot you want to use (you have four of them, the numbered buttons)
- turn the knob "mod source" until display says "keynumber"
- turn the knob "mod destination" until dispaly says "env 2 amt"
- turn the knob "mod amount" to set the strenght of the effect. With negative values, the sound will become more silent as you go higher on the keyboard, postitive values will make it louder. The higher the 'abolute' value, the bigger the volume jumps between the notes will be. As said, if necesary, adjust the setting for envelope 2. Different modulation sources add to each other, and cannot go above 100% (meaning, if you already turned env 2 (the amp envelope) amount to a high volume, you will not be able to add much "keynumber" tot reach 100% volume.
I hate to be the guy saying it, but with a synth like this I strongly advice you to read the manual. It is well written and easy to understand. To me, modulation is really what the P8 is all about, good to understand what you can do with the big amount of mod sources.