Force feeding should only be performed if the snakes life depends on it as a last resort. If you are certain your snake is gonna die if it does not eat, then force feeding might be the right thing to do. If a snake is emaciated (Starving/in very poor condition) force feeding might have to happen. 

Force feeding: Force feeding consists of getting the snakes mouth open, and forcing a small rat or mouse down its throat. It is best to have TWO people doing this, as it is very difficult to do alone. You must be very careful when opening the snakes mouth as not to hurt them, some snakes will open their mouth if you hold their neck, and others you will have to open yourself. When opening the snakes mouth, be CAREFUL, do not pry it open, instead use some tongs to slip in between the lips and shove it in, this should make the snake open their mouth, however if this does not work, you may have to use your fingers to open the snakes mouth, be sure to wear gloves while doing this. Now I suggest either a hopper mouse, or pinkie rat for force feeding, they are both small and you will be able to shove it down the throat easier than a big feeder. Be sure to shove it down the throat, and wait and see if the snake swallows it on its own, if it doesn't, shove it down further, but be gentle, do not use force. Once its down far enough let go of it with the tongs. If the snake begins to swallow allow it to do so, and stop shoving the feeder down. 

Assist feeding: This is when you help the snake eat. Sometimes a snake is so emaciated they cannot eat on their own. This is similar to force feeding, but not quite. You may have to open the mouth of the snake like I explained how to do in Force Feeding. In assist feeding, you may not have to shove it down the throat a far way down, just get in the snakes mouth and see if they swallow it on their own, if not then shove it down the throat a bit and wait. Note an emaciated snake will take longer to fully swallow a feeder than a healthy snake.

Syringe force feeding: Yes it is exactly what it sounds like. Using an appropriate sized syringe you can force feed a snake this way. There are different "formulas" you can use. You can blend your rat, or mouse up (Gross? Kinda), until it's liquefied, and slip it into the syringe, and then get the snakes mouth open using the techniques from Force Feeding, and shove the syringe in the snakes throat and push the liquid into its throat, it will swallow, or just go down. There are homemade formulas that don't involve blending a feeder down.  This can include eggs, chicken baby food, water (Ensure extra electrolytes added), and supplements such as Nutri-Cal. Blend these ingredients together into a liquid, suck it up with the syringe and syringe feed the snake.

 -Vosjoli, Philippe De. The Ball Python Manual. Advanced Vivarium Systems, 1995. (Citation)