I suggest the following :
- you need a bloc of hard plastic, or better metal (ali) deep enough, for instance 2 inches, to keep the bit straight while you drill. It must be large enough to allow it to be safely clamped to the transom
- you drill one perpendicular hole (or more) , with a static drilling machine, router or equivalent. The diameter of the bit must be that of the hole you want into your transom - say 10mm
- you punch the holes centre on the transom, and pre-drill them @ 5mm for instance, 5mm deep, to get a pre-centering hole
- now you drill them @10mm, 5mm deep
- you set the metal block in place, using the bit as a centering tool, and clamp it in place
Now you can drill the 1st hole through the transom.
When this is done, just do the same for the other ones.
I would suggest to add a plywood clamped together with the block, on the other side of the transom to get a clean hole.
Other suggestion : use an old 10mm bit first, because a new one will be ruined after a few holes due to GRP abrasion ...
I have a block specially done with 2x10mm holes, I used it to drill U-bolts holes into transom.