How to make a Hakama
Please note that this is not a historically accurate pattern; it's just what I figured out on my own. For a more accurate hakama pattern, visit AnneLiese's Guide to Japanese Clothing.
This is a basic layout of the pieces you'll need. It's not really to scale; you'll have to make the pieces longer or wider depending on how tall the person is and how full you want the hakama. The waistband should be long enough that it can wrap the waist 2 times. The waistband casing should be less wide than the back of the hakama. The waistband should be half as wide as the casing so it can go through it easily.
This description makes the assumption that you have some basic sewing experience and that you will hem or turn under all raw edges I don't mention.
First, measure and cut out the pieces. You might want to do a trial run out of tissue paper or muslin just to test the sizing before you cut your good fabric. Don't forget that the front will be pleated, so you'll need extra fabric there to keep the fullness of the bottom. You'll need more fabric depending on how deep you want your pleats.
Sew the two front pieces together from waist to crotch. Then mark the pleats for the front piece and sew them. Sew the two back pieces together from waist to crotch. Mark the pleats for the back piece and sew them. (Traditionally, hakama have seven pleats - 5 in front and 2 in back)
Lay the new front piece and new back piece on top of each other, with the right side of the fabric inside. Now you'll sew the insides of the legs together. How this works is you take the left side of the piece on top and sew its center seam to the left side of the piece underneath. Then you do the same with the right side. This makes it like a pair of pants. If you're having trouble figuring this out, just look at a pair of pants and see how the pieces are attached. ^_^
Still with the pieces face-to-face (inside out), sew the side seams together (front piece to back piece), but only along the straight part (from bottom hem to the bottom of the side vents). You will need to hem the side vents individually. These allow room to get into the hakama.
Try on the hakama to check the width of the front and back panels. You may need to make some darts or such in the back if there's too much fullness at the waist.
Once you've got the hakama fitted right, fold the waistband casing in half and sew it to the top of the back piece. It should be like a tube, so the waistband can go through it.
Fold the waistband in half, right side together, and sew it into a tube. Turn it right side out and sew it to the top of the front piece.
Once all of this is done, hem the bottoms and then turn the pants right-side out and finish up any hemming that still needs to be done. Press all the seams so they lie flat and look nice. You'd be surprised how many people forget about this step when making costumes...
You can either starch and press the pleats all the way down, or leave them soft and full; it's up to you and often depends on the kind of hakama the character you're cosplaying wears. To wear the hakama, step into the legs and slide the waistband ends through the waistband casing at the back and out again, to tie in front. This will keep your hakama from falling down. ^_^
Note: There is a more traditional variant of the hakama, which has ties at both front and back and a stiff board-like piece at the back waist. With this kind of hakama you tie the front ties first, crossing them in back, wrapping around your hips in front and then tying in back. Then you bring up the stiff waistband in back, center it, and tie the back ties, knotting them around the wrapped ties in front. The proper way to do it is pretty complicated, but as long as you have a knot in front you're ok.
Note #2: Hakama are meant to have a gap at each side so you can see the kimono underneath; they're not supposed to close completely on the sides. The gap can be narrow or wide, depending on the width of the hakama and of the person.