Yes, please be careful injecting meth. Shit makes my head spin even when I have a tolerance and it burns like a mutha if you miss. It would be best to snort or smoke it but if you're dead set on shooting..
It's like injecting anything else, but maybe use a bit more water than usual and push the plunger SLOWLY. That way you can abort the rest of the shot if its too intense. You don't wanna OD, very uncomfortable not to mention dangerous. You can always take more if you don't get enough.
If your veins roll try to go in from the side the vein rolls to, kinda hard to explain but I'm sure you understand. It helps to be in a warm room and use a tourniquet, always untie before doing the shot. And if you can't find a vein you can always plug it. In case you don't know what that means, you break the needle off and squirt it up your butt. Sounds ridiculous but it works.. I do it often. Just make sure you've pooped recently or the meth will soak into the turds instead of your rectum.