(Or, what I wish I knew about preparing fabric for sewing before I spent all that time and energy just to have to do it all over.)
But it's necessary.
Anytime you sew something that could find its way into a washing machine later, you need to prepare the fabric before it ever sees the cutting table. If a washable fabric is cut and sewn without being prewashed, it will pucker and bunch up oddly along the seams when the resulting garment or household linen is later washed. This is because washing causes the fabric to shrink, at least a little, so that the stitches in the seam fit into the fabric differently than they did originally. The fabric may even shrink enough that the item no longer fits at all.
Some fabrics should never be washed, but should be dry cleaned instead. These include velvet, rayon, most wools, and silks, as well as others. When you buy fabric at the store, the end of the cardboard bolt core should tell you what kind of care the fabric requires. Dry cleanable fabric needs no pre-cleaning before sewing, but, make sure that whomever is caring for the sewn item knows what care it requires.
When you're working with a washable fabric, it should be prewashed in hot water, and dried on the hottest setting recommended for that fabric. This is so that it will do as much shrinking as possible right away. If you get most of the shrinking done before the item is cut and sewn, the finished item will not likely shrink noticeably later on.
Wash the fabric with other laundry in the same color family, as you never know how colorfast the dyes used in the fabric are. You don't want to prewash your red cotton twill yardage with your white work shirts, at least not the first time. Similarly, you don't want to wash your brand new black Metallica concert t-shirt with your pale green linen.
Don't just toss the folded up yardage in the washer and call it good, either. Unfold it completely, and feed it into the machine lightly, so that it can move with the agitation. Don't loop it completely around the agitator, as this can sometimes cause the fabric to constrict around the center post, putting both the fabric and the washing machine itself at risk. A single length of fabric should not lie in more than half of the washing barrel.
When the fabric comes out of the washer, it will probably have twisted and tangled a bit, both with itself and with other laundry items. Detangle, untwist, and toss lightly into the dryer. Run the dryer until everything's dry, then remove the fabric while it's still warm. You can shake some of the wrinkles out at this point. Then it's off to the ironing board.
Ah, yes, the ironing board.
It's the rare fabric that doesn't really need ironing after being washed and dried for sewing. Every once in a while I'll skip this step, but only if the fabric is heavy enough to lie flat of its own accord. When you're laying out your sewing pattern and cutting your fabric, you need it to lie smooth and flat. Wrinkled fabric causes choppy, rough cutting, which can create enough difference in the finished item to make it not fit. So, yes, ironing is necessary.
If you don't have more than about two yards of fabric, this really isn't a big deal, but, I have ironed 10 yards of 108 inch wide muslin. That's 270 square feet of fabric. Let me tell you about suckage! If you do have more than two yards, put on a good movie, adjust your ironing board a little lower, and pull up a chair. You may as well be comfortable and entertained.
Set your iron to the temperature recommended for your fabric (most irons have a guide printed right on them). Since it's a washable fabric, steam should be fine, and will help get out the sharper creases. If your iron doesn't have steam (an antique!) have a spray bottle handy filled with clean water.
A note on water for steam irons: as a rule, you should use distilled water in your iron. Tap or spring water has minerals in it which will build up in an iron over time, clogging it up. If you find yourself out of distilled water at a crucial moment, using tap water in your iron once in a while won't destroy it, but you should refill it with distilled water as soon as possible.
Start at one end of your fabric, pressing it smooth and flat. Once the entire length of fabric is pressed, fold it in half lengthwise, right sides together, the way it was on the bolt. There's a trick to this:
Find the middle of the length of the fabric. Fold the fabric in half here, selvedge to selvedge, and pick up about a three foot section by the selvedges, so that the center fold is hanging. You will notice that the fold shows a bit of twist to it, so that if you were to iron it flat, there would be wrinkles at the fold. By shifting the selvedges left or right with regard to each other, you can make this twist smooth out and hang straight. Once it does so, carefully lay the center fold on your ironing board and press it in. Now, working from this point to each end, match up the selvedges and press in the center fold for the entire length of the fabric. If your fabric is really long, over six yards, you may have to do the hanging fold trick again, further along the fabric. You'll know if your fabric starts refusing to lie straight when the selvedges are matched. If this happens, you may have to re-press a little bit of the fold that you had already done, to make it flat. Don't worry, fabric is pretty forgiving.
That's it. Once you've finished pressing the center fold in, your fabric is ready to be laid out for cutting and sewing.