This is how I attached my skins for my R2-R9 Droid
& for Kaities R2-KT Droid
This assembly is using the aluminum Frame and John Sherrell's aluminum skins.

Things you will need...
#4-40UNC Spiral Hand Tap. Buy a good brand, not something from Job Lot.
3/32 inch (.09375 inch) Diameter Drill Bit [FOR TAPPED HOLE IN FRAME]
If you check any chart this isnt the recommended bit size for tapping this thread BUT
Because of the softness of aluminum and the small size of this thread
Going oversize will help prevent binding and your less likely to break the tap
1/8 inch (.125 inch) Diameter Drill Bit [FOR SCREW HOLE IN SKINS]
Hand Tap Wrench.
#4-40UNC x 1/4 inch to 5/16 inch long phillips pan head screws.
Phillips Head Screw Driver.
Clamps or someone to help you hold the skins in place
Cutting oil or Liquid Soap.
Can of compressed air
Safety Glasses

It's best to have the Utility Arms, Large Data Port and Shoulder to Body Hubs in place before you mount the skins.
This will help you to get everything aligned.
Position the skins in place. Check seams.
Clamp them down to the frame or have someone hold them in place.
I found a bag full of various size spring clamps at Home Depot for like $15. They work great!

With the 3/32 inch drill bit slowly drill through the skins
and into the frame. Hold the drill horizontally and try to get the hole drilled center of the frame ring side.
Go slow, let the drill bit do the work.
If you force it you'll break the bit or drill an oblong hole.
Drill as deep as possible. This will make tapping easier.
You don't want your tapping bit to bottom out in the drilled hole.
If it does you'll have a better chance of the tap binding and snapping off.
Clean out the hole by blowing into it a few times or
use a can of compressed air.

With a 1/8 inch drill bit slowly drill through the skins only.
This is a clearance hole for your screw.
If you find later that your skins need tweaking, you can always make this hole slightly larger.

Now the tricky part!
Apply some cutting oil, or liquid soap to the #4-40UNC tap bit.
Slowly tap the hole in your frame ring.
Turn the bit until you feel it take bite.
Then keep turning slowly!
If you feel ANY resistance slowly back out the bit.
Clean the bit and hole of any debree.
Start tapping again. Continue till you've got most of the hole threaded.
You can test this by screwing in your hardware to make sure you have deep enough thread.
Tapping a thread this small isnt easy. The tap can break easily!
If it does break your only option is to try a new spot and start again.
They sell tap removers but they very seldom work.

Drill and tap as many holes as you feel are nessasary.
Then screw your skins on
For R2-KT I used pan head screws
For R2-R9 I used plastic head thumb screws
This made removing my skins easier, I don't need to have a screw driver on me.
So if I'm at a show and someone wants to see inside, a few finger twist and the skins off.