In terms of dimensions, some commentators** hold that there is no real minimum for a TK, and that any dimensions are OK. The Mishnah Brurah*** says that the TK must be 3/4 of an amah (15ish inches) wide, and others hold that it needs to be an amah (about 19 inches) wide. Depending on how you hold (I’m not going to posken for you), and how you’re built you might wind up with a garment that’s not so close-fitting, in which case snaps or hooks (see below) would be extra helpful.

Okay.

1) Take the garment, and using the scissors, cut the thread/stitching on one side of the garment from the bottom, so that you’re undoing what’s holding the two pieces of fabric together. Go slowly so that you don’t accidentally cut the garment itself. Cut the stitching ’till your side is more than halfway open (the halakha demands that the “majority” of the garment be open in order to be defined as a corner, so 51% open is the minimum.) I reccomend keeping some space closed at the top, so that it will fit better. Do the same thing on the other side. Now you have a garment with four corners.

2) Put in a little bit of stitching on the sides, where you’re done opening the garment, to make sure the thing doesn’t keep opening of its own accord. A couple of good stitches should be enough to keep it together.

3) Take the fabric, cut little squares an inch or two squared, and stitch them onto the corners of the garment (like how they have on a tallit gadol, aka regular tallis). Best to fold down the edges of the fabric inward, so the frayed bit is tucked in.
This will reinforce the corners and keeep the tzitzit happier. If necessary, though, this step can be optional.

4) Then, using your scissors, cut a small hole in the corner, in the middle of the fabric square–5cm in from the bottom and 5 cm in from the side. Make sure it’s big enough to get four tzitzit strings through, but ideally not a lot bigger than that. Then, finish the hole by stitching around it (using the thread to pull the hole tight and stay put.) Looking at the holes on the corner of a regular tallis or a store-bought tallit katan might give you an idea of what you’re shooting for. I don’t know any fancy sewing terms, so I can’t give you one to describe this. It’s not rocket science–do this to make sure the hole is not going to get any bigger or torn through, and that the fabric is sewn to the garment.

4b) Technically speaking, you could sew on a hook-and-eye or snaps to sides of the garment so that it closes in a “non-permanent” way and thus fits closer to your body. I personally have never found the need for this, but it’s an option.

Now, you should have a kosher garment onto which you can attach tzitzit! Cool.

5) Then, tie the tzitzit. here are some good instructions, from The Jewish Catalog, and here are some more instructions, with the illustration from The Jewish Catalog. If you’re tying with tekhelet (a special blue thread), you can go here to see all the different fun and freaky shitot that different poskim have used, plus instructions on how to tie thusly.

And that’s about it! Before you put on your new beged, say the bracha “al mitzvat tzitizit” and, of course, “shehechyanu” for your new garment, and get your fringe on down the street.

Awww, yeah.

*For info on women taking on positive time-bound commandments, check out my post here about it. Or, if you’d prefer, some sources for your entertainment: Mishnah Kiddushin 1:7; Tosefta Kiddushin 1:10; BT Eruvin 96a; Rosh Hashonah 33a; the stam in Sifrei Bamidbar 115; Tosafot Brachot 14a, Tosafot Kiddushin 31a (re: Baba Kama 87a); Rambam Hil. Tzitzti 3:9; Ran Perush L’Rif Rosh Hashonah 955; Rema Hil. Tzitzit OH 17:2; Rema OH 589:6; Iggerot Moshe OH 4:49; Tzitz Eliezer 9:2; as well as Rashi, Rebbeinu Tam, Rashba and others.

**Aruch ha-Shulchan 16:5; Hisorerus Teshuvah 3:38. See Igros Moshe Y.D. 3:52-2 for an elaboration.

***Hilchot Tzitzit 8:17; 16:4.