Easy, you break the top off of the amp. CAREFUL, use a cloth or something else to hold just so you don't cut your hand. With the top broken off, insert the needle into the amp (right side up) and draw out the fluid. That's it bro!
I wouldn't drill into it. That would leave little glass shards in the oil, which probably wouldn't feel good if you injected them. I've seen "amp openers" on some pages for sale though that would probably make it easier.