The first step to any pants role is (of course) chest binding. There are many different ways you can choose to wrap up the girls, but I'll give you the most common choices.
Sports bra: You can wear one sports bra or layer two on top of each other. The best type for this are the seamless, unpadded bras that have no underwire or any type of molded support. Inexpensive bras usually work as well as expensive ones for this purpose, so this is another cheap way to chest bind.
However, this method is not super-effective for bigger-chested women and can be uncomfortable in general. Like any method, this one would take some experimenting to get the best result.
Compression vests/FTM chest binders: This is the most expensive but, in my opinion, the most effective method. There are many different types of vests/binders that you can buy (there is a very good review blog here), but my absolute #1 choice is the High Performance Velcro Long Binder from LesLoveBoat. It is extremely comfortable, washable, ventilated, and it doesn't interfere with any of the muscles needed for singing. It was a little expensive, but completely worth the investment. It came very fast in the mail and exceeded my expectations. There are other, less expensive, options on that website that are worth checking out as well.
Be careful to always have healthy habits when it comes to binding. Never bind too tightly (it should never hurt) and don't bind for long periods of time.
Here are some basic rules for binding safely:
1. Bind using only an actual binder (make sure you measure and get the proper size), or tight-ish sports bra in a pinch. You want something that compresses while still having give so that your lungs can expand.
2. Never use ace bandages/belts/tape/etc to bind.
3. Do not sleep in your binder.
4. Do not wear your binder for long periods of time. I’ve seen 8 hours mentioned as the max amount of time. Learn to read your body. Some people cannot handle binding for even 8 hours but ignore the signs and injure themselves.
5. Whenever you take your binder off give a few big coughs, this well help to break up any fluid that may have settled in your lungs.
(safety rules courtesy of thatsnothowitworks.tumblr.com)
For more tips on binding, you can find additional information here, but some of the links are outdated.
Creating an Illusion - The Basics
In general, the best way to appear masculine onstage, in terms of dressing, is to create the illusion of a masculine silhouette. This means not only binding, but also selecting the correct size and type of shirt, pants, shoes, and accessories. The idea is to create a straight line from your shoulders down to your hips (a man's waist) and down through the leg. An excellent example of this can be seen in the Jennifer Rivera video I have included in the Basic Movement section of Learning from the Masters.
Below are pictures of a successful masculine silhouette created on a feminine body.
Dress in Rehearsal
What you wear in rehearsal is very important to the success of your role. First of all, you should always wear clothes to rehearsal that you can move in, but you should take special care to wear clothing that allows you to move as a boy. Personally, I find it distracting and difficult to do staging rehearsals for a pants role if I'm wearing a skirt or girlish shoes. Pants are often best.
I find it extremely helpful to bind for rehearsal. This helps you feel how to move and sing as you will have to in performance. Also, binding changes the way you move and breathe: learning early on what this will be like is much easier than trying to throw it in at the last possible second. This also seems to make things easier if you're playing a romantic role and your colleague must physically relate to you (more on that subject in the movement section).
It is sometimes helpful to bind in the latter stages of learning the music. When I'm practicing on my own, I spend some of my practice time singing with the binding on so that I can adjust to my breathing.