I've not done a D-70, can only rely on past experiences with D-50's, D-10 JV-80, Korg M1's..

I tend to mark the faulty keys with electrical tape before hand.

Lay the keyboard upside down, on a bed or sofa if possible.
Access is usually though the bottom cover, so remove the necessary screws (keep them in a small container).

On D-50's, and the Korg M1, you need to remove some circuit boards to allow access to the actual keyboard. Roland do tend to colour code their connectors so that will help. Take a picture if you want to remember what goes where.

Next, we need to remove the keyboard assembly form the rest of the synth.

Once removed, you will need to remove the some of keys from around the area effected area.
On the D-50 this means removing the springs, and a plastic strip on the backside of the keyboard. The notes should pry out.

Hopefully you will now see a rubber contact strip. You may find it has lifted away from the circuit board, normally where one strip meets another.
You can either strip it back a few notes, or remove notes to allow you to remove the whole contact strip.

I tend to use a cotton bud to clean the carbon contact of any dust and dirt. If you want to use any contact cleaner, spray it onto the bud, not the contact.
I tend to do that on the contacts of the PCB.

When you re-assemble, make sure the contact strip is nice and flat against the PCB.

Replace the keys (black notes will need to be replaced before the white notes).
If there was a small plastic strip, that will need to be re-glued with a simple tacky glue (not super glue), I would test the keyboard first.

Re-fit the keyboard assembly, circuit boards a few screws just to hold the boards in place, and cables.
Flip the keyboard over, plug it in, and give it a try.
If all is well, then unplug it again.
Tidy up inside the synth, remaining scres for circuit baords, cable ties on cable looms.
Fit the bottom cover..

Hopefully job done..