"Blocking" is the process of stretching your finished project into the desired shape, then wetting it down or steaming it so it'll stay that way. If you've managed to knit or crochet an acrylic beanie that's a little too small, blocking it is also the best way to stretch it just enough to fit. That said, the acrylic content means you should be extremely careful about using heat. Wet-blocking without heat -- as opposed to steaming -- is the safest method.
The General Blocking Process
Here's how wet-blocking works: You either soak the beanie in a bowl of water until it's saturated, then press the excess water out on a towel and pin it into the desired shape and size; or you stretch the beanie and pin it down first; then saturate it thoroughly with a spray bottle. After it's soaked, leave the beanie in place until it's completely dry.
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Because beanies are relatively small and since you're actually trying to stretch the piece to a larger size, the bowl of water method is usually easiest. Make sure you don't wring the beanie to remove the excess water; lay it flat on one end of a towel; fold the other end of the towel over it, and press down to remove the excess water without distorting the stitches.
About Those Pins
The easiest blocking pins to work with are "T" pins; the crossbar makes them easier to place. Start by pinning the brim or hem of the beanie, placing the pins close enough together that the brim makes a straight, even line. You shouldn't need to place any pins on the crown or sides of the beanie, but, if you see any stitches that seem stressed or stretched out of shape, now's the time to place pins around them to help ease them back into a regular appearance.
Using a Head Form
You could block your beanie out flat, but, since any stretching you do during the blocking process will be permanent, and you want it to fit on a human head, you should use a block form that approximates the shape and size of your head. A large foam ball, or a foam bust that approximates the shape of your head, is ideal. If you can't find anything else, you can even drape a thick towel over a pumpkin of the appropriate size and stick your pins into it.
The Steam Option
If you really want to steam-block your beanie, you can: Pin it to shape; then hold a steam iron near enough to it for the steam to relax the stitches. But you're running a very real risk with the product of your hard work. If you let the iron touch the acrylic at all, the yarn will melt. Depending on the heat of the iron and the delicacy of the yarn, getting it too close to the beanie can have the same effect even if it doesn't actually touch.
Checking the Size
Wait until the beanie is completely dry before you remove it from whatever you used as a head form and check to see whether it fits your head. You can always re-block the beanie to stretch it more, but there's really no way to reliably undo too much stretching. So when in doubt, always err on the side of less stretching.