These steps are only necessary for unusually large prey, as smaller (Or, especially, shrunken) girls are generally fairly simple to swallow.
First, you must make sure you're ready for such a big meal. You'll want to start on smaller things. Start by swallowing small animals, then larger items such as watermelons and such, until you feel you can swallow them easily and digest them comfortably.
Once you are confident you can hold her down, you need a plan of how to actually eat them. If they are a willing target this is easy, but if they are like most girls they don't fancy the idea of being ingested by a complete stranger and will likely be kicking and screaming all the way down. Though some preds prefer it this way, it definately takes some experience and a lot of brute strength.
A simpler method is to use a sedative or paralyzation agent. Cholinesterase works well, as it deactivates their muscles without taking away their sensations, but a much more fun agent is Struggle-B-Gone, available at Duamutef Enterprises Inc., which paralyzes the target for a short time, but wears off upon contact with any acid so you can feel them squirm. Heh heh.
You will want to select a safe spot to strike, one in which you can enjoy your prey and swallow her at your leisure. Her own apartment is ideal, provided she lives alone, but a hotel room or public bathroom will suffice.
Once you have sedated your target, you will want to begin gradually, generally at the feet.  The best position for this is with your prey raised off the ground by a few feet, but you can just lift her legs if this is not possible. Whether she is naked before you begin is a matter of preference, but I would strongly recommend removing her shoes and any jewelry she may have. 
Spread your mouth wide and place her toes at the back of your throat with her heels against your tongue. If you've positioned yourself correctly, you should be able to move forward and your prey should begin to slide easily down your throat.
Just keep sliding her down in this fashion, swallowing when you can. You will generally need to unhinge your jaw or stretch (Depending on your particular skills) once you pass the calves or knees, as the fit will become tighter and more difficult.
Once your lips pass the buttocks, you will want to tilt your head back and relax your neck and jaw muscles. If all is going well, you should feel her feet pressing against the bottom of your stomach. Just relax and swallow, letting gravity pull her into your esophagus.
Generally, her legs will naturally  bend and fold against your stomach as she goes in, leaving more room, but you may need to coax them to do this. It generally doesn't matter which way the knees are pointed, so long as they aren't straight.
Once you reach the breasts, your stomach will most likely be full enough that you cannot simply rely on gravity to help you swallow. Your throat will be very, very stretched by her chest and arms at this point, but this is normal. If you have a particularly long tongue it can be used at this point, but for most people it is most expedient to grab her shoulders and/or head and begin to shove. This can be a bit of a chore, particularly if she's much larger than your throat, but it will be worth it. You want to time your shoves with your swallows so each one shoves her further into your belly with as little effort as possible.
You will want to continue swallowing in this fashion until the tip of her head is just passing the inside of your teeth (Her chin should be pressing against the back of your throat). At this point, take a deep breath, concentrate, close your jaws tight and swallow HARD.
This should shove her all the way down your throat and into your belly. If it does not, just continue to swallow hard until you have her.
Once you have successfully engulfed your prey, you will notice that your belly is enormously distended, and it is often very difficult to move both due to the extra girth and the extra body worth of weight on your legs. It is generally good to have a place closeby to stretch out and relax. Your victim's own bed or the bed in the motel room will function nicely, but personally I find nothing is more satisfying than sitting out on a balcony, stretching your legs as your belly sags and gurgles in the warm sun.
Digestion generally takes anywhere from mere minutes to days at a time, but in most cases it takes from four to thirty-two hours. If your belly is still stretched at the end of a few days, you may want to try some over-the-counter digestive supplements.
This leaves you only to enjoy the sensation of your prey being churned in your stomach, squeezed through your intestines and passed through your bowels.
(You can expect to be on the can for at least twenty minutes. Just make sure you have a plunger!)

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