I read like crazy before I popped my first amp. Got great advice from all the bros here.
I'm still in my first cycle, so I'm still new enough to remember everything I read. My test comes from Nile Co. -- notoriously thick glass and no pre-scored amps
Here's my procedure:
1. I have a small metal file I use to score the glass around the neck of the amp (the Nile's have no pre-scoring at all). I take my time and get a nice, even scoring all the way around. It takes a little while with the Nile's. You can get a decent file at any hardware store for about $5.00. Mine is rounded on one side and flat on the other. I use the edge of the file where the two sides come together. As you go, you'll FEEL the edge of the file score through the glazed outside of the amp glass. You'll SEE the line you score into the glass - it should be easy to see. TAKE YOUR TIME. And remember - how well you score the amp determines how cleanly the top comes off. You don't want any chipping or anything like that. Some guys have said they use a serrated knife to score, but I had NO luck with that method.
2. I kind of scrunch a paper towel to hold the lower half of the amp and protect my fingers in case the amp should shatter (hasn't happened to me yet).
3. I use the removable cap from a pen, place it over the top of the amp and SNAP - the top comes off clean every time. The key is to make sure you get a good scoring around the neck with the file.
4. I open a new syringe, get the pin into the test - careful not to let the pin touch the sides of the amp - and draw my gear nice and slow.
5. If you can't draw your gear without your pin touching every forign object in it's path, be sure to change pins before you inject.
This is probably WAY more info than you need, but any newbie might read this. I didn't realize it would be so tricky to open the Niles before I got them, and when it was time I couldn't wait for an amp opener to come in the mail.
If your amps come pre-scored, you need to get advice from someone who has experience with them -- I've only opened Nile's.
Good luck