I started IV'ing meth in January of this year after having smoked it for the first time in October of 2009 and quickly saw my life as I knew it come to a screeching halt in early June. I lost everything, house, job, car, dogs...I would highly recommend leaving this drug alone, but if you're going to mess with it, IV is the cleanest delivery system, just make sure you are experienced in shooting and have plenty of discipline when it comes to drug use.
IMHO the following would be decent guidelines for your posted questions:
How much should one person take at a time?
Depends on indididual tolerance, quality of dope, ice, etc - if you're new to meth, a good starting shot might be 1/8 of a gram (.125). If your tolerance has increased due to frequent use, perhaps 1/4 gram (.25 - typically a purchased "twenty" in the US). Always test your dope prior to injection (especially when you obtain from several sources), to test: roll a small bowl in a piezo first, or run it on foil. Quantities just go up from there.
How often should you do a shot?
The rule of thumb prior to doing a follow-up shot is as follows: if you can't hit yourself then you're probably still high, no shot needed. Probably 4-6 hours after your first shot, but as stated above, if you can't hit a vein then you're too high to have another shot.
How do you know when you have been 'smoked down'?
I'm not exactly sure what you mean by "smoked down", perhaps you can explain this. Not a known term in meth circles.
When is it time to up your dosage?
If you're not getting ANY rush from your shot and you haven't missed, you probably need to up your dose.
What are the steps in preparing a shot of ice? Water - Dope ratio:
I've always used the same formula for prepping a shot of meth: whether I'm prepping a single shot or a shot for two, I ALWAYS use the same amount of water -40 units. That way I always know what comes back after I draw up. If the mixture is too thick, draw up a little more water into your syringe.
As far as method goes, put your meth in a spoon, add water, crush/stir so there aren't any chunks, cotton/wheel filter, draw up, bang away.
I can't stress enough that it is CRUCIAL you do NOT miss when injecting meth. Lord knows I missed plenty when I first started, unfortunately one of my misses ended up abscessing and getting infected. I had to be treated with IV antibiotics to cure the abscess, and I'm left with a gnarly little scar that'll probably never go away. Also, make sure that EVERYTHING that comes into contact with your dope is ABSOLUTELY CLEAN. Your spoon, your syringe, your water (tap water is fine) should be as clean as possible, that's just common sense.
Hope this helps
MsKitten
I was never a methamphetamine user but I was a heavy amphetamine user back in the day. My paranoia was off the CHARTS when I was abusing. Fucking eating white bread and water huddled in the corner of my room at 4 am because I think I can hear people talking in the other room (there wasn't anyone in the other room.) 
