Herry,
This is a 2 person job. To mark out the area you want to drill, keepin in mind the placement of your back armor eyelets...first, get your friend to place the JP onto your back [with your back armor on], note and mark out the location of the back armor eyelet to your JP.
Next, taking into consideration the sized buckles and straps you are using, mark it down and draw out the placement holes.
[Remember, this part of the set up is important if you want the JP to sit perfectly on your back, with secure stability and very little movement restriction, so take your time with this.] It took me 6 times to work out the proper placement of my drilled holes and it was a bit tedious...but it paid off in the end. I have one of the most secure and stable JP setups around. Even with the dancing I did in Taipei, my JP stayed put on the same spot on my back.
If it's all properly marked out, the holes you'll need to cut will be on the plate that's removable. Then use a dremel with the drill bit attachment, drill out the edges, then switch to a fiberglass reinforced cutting wheel to remove the big pieces. Once done, use jeweller's hand files to clean up the edges and fine sand it to smoothen out the raw areas. [FYI, you should have cut out the holes first before painting.

].
I would also suggest you do a bottom securement as that will help keep the JP in check and position.
BTW, hit either Suntec or Shaw Towers for the High Seirra backpack. It's no longer in black, but multicolored. Still, no ones gonna see it as it sits under both your vest and back armor.
Good luck!
Tim.