I'm starting with a top that is almost completed. I've sewn the front and back together, and added the sleeves.
As you can see, I've cut a hole where I want the keyhole to be. You can add a keyhole to the front or the back, in this case I'm putting it in front. You can do a pretty large keyhole in the back of a top, but keep in mind how much skin will be showing if you're putting it in front. A more modest hole for the front is probably best. :)

Here is a close-up of the keyhole opening. I've cut a slit where the keyhole meets the collar of the shirt.

When cutting the keyhole opening, you should also keep in mind that the keyhole will be a bit larger than the opening you've made, depending on how much of a seam allowance you use. So always cut your keyhole a litle smaller than you want it.

You will also need a piece of fabric to trim the keyhole.

They key to keyholes (haha, get it?) is very stretchy trim fabric. The stretchier the fabric, the better your keyhole will turn out. If you've tried keyholes before and the fabric bunched up around it, your band of fabric around the edge probably wasn't stretchy enough.

For this top, I'm using a piece of rib knit. This stuff is PERFECT for keyholes, because it's insanely stretchy and elastic- it doesn't lose it's shape when stretched a lot. It's also called ribbing, baby rib, etc. It's the same stuff they use for the cuffs of sweatshirts.

So back to the keyhole- you will want a piece of trim fabric that is equal in length to the circumference of your keyhole. You can estimate on this a little, but always err on the side of longer for the trim. We'll say my keyhole and my trim fabric were both 14". Your trim fabric should also be double the width you want it to be when it's finished. I want 1/2" of trim, so my piece of fabric is 1" wide (plus a seam allowance!).

You can use lace or some sort of ruffly stuff around a keyhole, but it's tricky. You'll probably want something that is elastic in the middle with ruffles on both sides. In this case, you won't need to worry about the width.

Fold the trim fabric in half, and pin it to the right side (outside) of the top. This is kind of like making a stretchy fabric waistband, or a crew neck style collar.

You'll want to be sure to stretch the trim as you pin it to the keyhole, and again when you sew around the edge that you've pinned.

Now it's time to decide how you're going to finish the top.
If you want a closed keyhole, baste the two edges of the keyhole together (red line in the photo), and then finish the top by adding a collar or hood or what-have-you as you normally would.
If you want an "open" keyhole that you can tie closed, don't baste the two edges of the keyhole together. Instead, start at one end and sew the collar or hood on until you've reached the other end. Then you'll want to add some strings or cord or ribbon to tie the keyhole closed.

I wanted a hood with a keyhole that could be tied closed. Yay!