This is not coming from an educated expert, just a common user who did not have any experience with the meth pipe at all in the beginning having always snorted or ingested speed base previously. Just thought I would share my technique and let others comment to either confirm or argue against what I have learned over my time with this 'interesting hobby'
The Sweet Puff (or less romantically - The Meth Pipe) It's a tube of glass varying in length from about 20cm down to a stumpy 8cm or so depending on where it came from. The tube should curve 90º at the end and terminates in a larger round bulb with an opening usually around 1cm in diameter. It's illegal to buy or own one where I come from but is still readily available at many tobacconists and 'sex' shops that sell smoking paraphernalia, under the counter of course. In Australia they generally sell for $30AUD
Preparation Assuming you've just acquired your pipe, as well as procured the smoking agent (from a reputable supplier one would hope), you're going to want to know how to use it. This guide to help minimise the pitfalls of trial and error. Firstly after you've unwrapped the apparatus, I'd recommend giving it a visual check to ensure there are no foreign bodies lodged in either the bulb or stem. There shouldn't be, but it cant hurt to check anyway.
Loading It appears to be a contentious issue as to how much you should actually load into the bulb. Personally I think its up to the user, but as a general guide I'd recommend .1 of a gram. This might seem daft, but a common question that newbies ask is how to actually transfer the shards to the bulb. It commonly arrives in the smallest zip lock bag available and a normal teaspoon wont fit. The best tool I have found for the job is to take a normal plastic drinking straw and cut it at a 45º angle turning it into a very effective scoop that can fit into small spaces. The wider sort of straw such as ones you get at McDonalds are ideal but any will do. I've heard that its a good idea to crush up the shards and shake the bag up to even out the purity but I don't personally practice this. I enjoy watching the shards boil and melt
Heating We are not trying to burn the crystal as though it was a cone of pot. Meth has a much lower boiling temperature and can burn easily if you are careless. Meth vapour = good. Burnt meth = bad. I use a normal BIC butane lighter (stock up on them!) although I also keep a candle handy just in case I run out of gas. With your shards loaded in the pipe, hold it in one hand with the hole pointing upwards and with your other hand hold the lit lighter underneath the bulb. Do not let the hottest part of the flame touch the glass! If you don't know the hottest part is just below the very tip of the flame. Hold the tip of the flame with at least a 1cm gap between it and the bulb.
Even lower is better but it will take longer to vaporise and you will go through your lighters much faster. After approx. 10 seconds the bulb should reach the boiling temperature and the ice will start to melt. Expect an initial 'fizzing' sound and you might notice very tiny bubbles appearing as the shards liquify. At this point vapour should start to appear. Allow this to happen for about 2-3 seconds and the vapour will begin filling the bulb (and will also quickly begin to escape through the hole so timing is important here) Allow all the shard to liquify before you start to inhale
Inhaling Once there is sufficient vapour, exhale first and then draw back on the pipe. You will not require much force to do this as there is zero resistance in the pipe, and if you suck too hard you risk inhaling the heated meth directly which will a) leave a bad taste and b) not be absorbed correctly and will be a waste. At this point you will still want to be applying heat to the bulb. With the pipe in your mouth, gently rotate it so that the bulb rocks back and forth across the flame. It is said that meth best vaporises as it is cooling, not heating so you will discover that you will get a bigger cloud as you move the ice away from the flame.
Draw back for as long as you can, to inhale as much vapour as you can. Once you are just about there, you can take the flame off as the meth will continue to vaporise. Do not hold it in your lungs for more than a couple of seconds. The impurities that come with this stuff are not good for your respiratory system and the methamphetamine itself will absorb almost instantly (reaching the brain in 3 seconds - citation needed!) So the sooner you purge it out the better. Vapour will continue escaping through the hole so it is a good idea to cover it with your finger, or if the bulb is too hot to touch I'll sit the lighter over it. At this point now you will be able to continue inhaling the vapour without the need for the flame as the bulb will be hot enough to continue the reaction. Keep drawing out the vapour until it stops coming and there is just a clear 'puddle' in the bottom of the bulb.
Cooling A mistake I kept making was reapplying the flame to the bulb while the meth was still liquified. I'm not sure if this is common knowledge, up for debate, or just plain wrong so please correct me if you know otherwise. Putting the flame back on the already melted shards will cause it to overheat and will start to burn. What you should do is heat the meth till it melts, inhale as much of the vapour as you can, then take the flame off and allow the bulb to cool. Once the meth has dropped below its boiling temp you will see it recrystalise or 'freeze over' which is a common term used in my area - hence the name Ice perhaps?
Once it has done this it is okay to then reapply the heat and repeat the process. If you burn the meth it will start to turn brown and leave a very nasty aftertaste. To speed up the process some people will keep a damp cloth handy and dab it on the underside of the bulb. This may reduce loss through vaporisation but it also weakens the glass and yes they do shatter so be advised.
Resources: http://www.bongsmart.com.au/41-sweet-puff https://drugs-forum.com/forum/showthread.php?t=235835