IV vitamin C - which I believe is usually a sodium ascorbate, rather than ascorbic acid. The sodium variety is supposed to be easier on the system. I wouldn't want to be injecting a high concentrated solution of ascorbic acid, **** would probably burn and possible do some real harm.
That aside, there are some pretty miraculous studies on the use of IV Vit. C (from curing cancer to relieving opiate withdrawal)
I believe the main reason is to avoid the so called "bowel tolerance" of oral use vit. C. IV allows you to use ultra high amounts without the "intestinal distress" we'll call it.
Vitamin C foundation is a place to start.
Oh and to answer the OP's question...
I'm sure there are youtube's about this, for IV you want to find a vein, something you can hit. Most people use the pit of their elbows. Once you've got it in there, you want to draw back - if you see blood then your good and you can proceed with the injection. If you don't see blood, you're not in and your going to miss.
Remember. Stick. Draw blood before you plug. Good to go.
Not impossible. Might want to practice with water or something to get the hang of it - cuz I'm pretty sure IV C is kind of expensive.
B12's I've only heard doing them in like, a pinched roll of fat. Which is basically a pinch and stick operation.