OK, it isn't safe, but i've been doing it for nearly thirty years on a regular basis and i'm the healthiest person i know.
My list is at the bottom of this post, but please read all of this message first.
The trick is to go slowly and gently, and to stop when it gets even slightly painful or feels bad, or you feel nauseous. The risks are: rupture of organs leading to peritonitis, which can be fatal, or tension pneumothorax, which again can be fatal; bubbles entering the blood, which can cause sudden death; if you use liquid, water intoxication; if you use alcohol, alcoholic poisoning; ruptured blood vessels due to shifting blood away from the area of distension, meaning haemorrhage or a stroke, which again can be fatal; choking on your own vomit, where the acid in the stomach ends up eating away at your lungs making breathing permanently impossible and causing death by shock; excess pressure on the heart, leading to overcompensated heart failure, loss of consciousness and death; ruptured eardrums and deafness.
Having said that, this is what i've done:
Be really careful.
Mostly, i swallow air. I relax, lie on my back, take a deep breath and then breathe out gently and slowly while keeping my mouth and soft palate closed. Make as if to say a sound such as "P" or anything other than a nasal sound, but keep your mouth closed, then breathe out slowly until you get a gulping feeling. This is the first lot of air. Keep doing this until your lungs are empty or you need to breathe. Then, relax for a few minutes and think "expansive" thoughts. The air will start off filling your stomach but will gradually bubble down into your intestines. A few minutes later, your stomach will be almost empty and you can repeat the process. I have done this up to four times in a row. You eventually get an almost irresistable urge to fart, and you become very bloated. Each time you do it, you probably get about a gallon of air down, so if you do it three times, you will end up with around twenty-four pints of air inside you. I usually also breathe in after doing it, adding about one more gallon. It's best to do this, and everything else, on an empty stomach.
Incidentally, i once did this with cannabis smoke and it made me extremely high because it was all inside me and being absorbed, which was a truly awesome experience.
Other things i have done include taking a balloon, blowing it up quite big and letting it down into my mouth, gulping the air down in a similar way. This is quite exciting because after you've done it you can look at the empty balloon, remember how big it was before you started and think that all that air inflating the balloon is now inflating you. However, you have to be careful with this technique because it's difficult to keep the air out of your inner ears, which could perforate your eardrums and cause deafness.
Finally, you can actually use a pump. I have used a footpump, a bicycle pump and a stirrup pump. This is gentler than the other methods if you use one with a small stroke volume, and you can go quite slowly. A bicycle pump has a volume of around 2/3 of a pint, so it goes quite slowly, but i have used a foot pump with a stroke volume of 5 litres, which is around a gallon. This was intense and exciting, because you bulge and stretch dramatically, but also probably quite dangerous. In this method, you stick the pump hose in your mouth, hold your breath and put a clip over your nose, then start pumping slowly but steadily until you need to take a breath. I've achieved the largest inflations with a bicycle pump because the small stroke volume and gradual introduction of air gives your body time to adjust. The most i've managed with this, or any other, technique, is fifty litres of air, which is around ten gallons. However, it doesn't make you absolutely huge because what actually happens is the pressure builds up inside you.
Eventually what happens is it gets difficult to breathe because your distended organs push up against your lungs, and the pressure crowds the heart, which must be dangerous. However, the same things happen in pregnancy, although in that situation the hormones do things to help the body adjust to the expansion, which you don't get from inflation with air as far as i know, although it would be interesting to find out. Another thing that happens is that your arms, legs and head feel swollen because the air pressure pushes the blood out of your body and the carbon dioxide build up widens the blood vessels.
Afterwards, you burp and fart the air out. This is usually the most painful stage for me, and i've been really worried i'd done myself serious harm because it was so painful. It's not so bad if you have a warm bath while you're doing it though.
As i said, i do think it's dangerous and i do it myself because it's basically my life and i'm medically qualified, so i know about warning signs and can do something about some medical emergencies, but i wouldn't dream of doing it to anyone else because of the lack of control and sensory feedback. I could end up killing myself, but on the other hand i've been doing it for nearly thirty years and i am very fit and healthy.
Anyway:
1. Your own breath, a balloon or a bicycle pump. Some people use an aquarium pump anally. I haven't tried that.
2. Don't use water or any other liquid except maybe for small volumes of fizzy non-alcoholic liquid. It could dilute the salt in your body and could easily kill you. Also, liquids are not compressible so there is no safety margin, i.e. it will take up the volume it has and that cannot be reduced, unlike air.
3. I know most people do it anally, but i've never had much success that way, probably because my sigmoid colon has an unusually sharp bend at the top. I would always say do it orally, because your digestive system works in the same direction as the air comes in. I find doing it anally disgusting, unhygienic and it feels unsafe.
4. It isn't safe at all beyond a certain point, but then again if you think about the most flatulent you've ever been or the most stuffed with food you've ever been and holding your breath at the same time, that's probably pretty safe, but you can go beyond that and it isn't clear where the danger point is. Also, your body can adjust over time to more inflation if it has to, like most other things.
Honestly, inflating yourself isn't safe at all. Things could easily rupture and the like, so it's best just to leave it up to fantasies.
Well that, or use a trash bag or loose clothes or latex or what have you.