I've been working on a pattern for several weeks now and it's much more challenging than one would think.
The easiest way I've come up with is to lay out a 12 ft piece of silnylon, then using a Treking pole, set to about 45 inches or so, Tie and put the pole on the mid point of one long side, and tie it in place with a long guy line.
Stake out the corners with some pinned on Grossgrain webbing with the one midpoint up.
Take a 2x4 or some other long straight edge and using a sharpie, draw out the rough pattern of the back and two short sides. Once the lines are drawn, then take down the tarp and cut along the lines. remember to leave material for the hem and in case you didn't measure just right.
Take two smaller pieces of sil-nylon, trace the triangle from the midpoint on the long side to the backpoint you just cut to the back point of one of the short sides. Then take another piece of Sil and trace the other triangle on the opposite side. Sew these two pieces to the front, meeting the two pieces at the midpoint on the long side.
Sew on a zipper between these two pieces up about 36 inches from the bottom, then sew a seam between the rest of the two pieces up to the midpoint on the long side.
Using the pattern from Flying Brian's dad, Trail Dad Robbinson's website for his poncho, make a hood from sil and sew it on the tent you've created with the midpoint on the long side being the center point for the hood. Trim out the material within the hood on the tent, then sew a french seam on the hood joint to maake it smooth against your skin.
The toughest part will be creating the six sided webbing piece to support the treking pole and placing the hand slits along the seams you sewed between the doors and the tent body.
Personnally .... I found it much easier to just sew a long poncho tarp and set it up in a trapezoid shape with the back to the weather. You can even sew a beak for it, but the angles are sooooo much easier. Using 2nds silnylon from Thru hiker that will run you about 6 bucks a yard, 3 yards for the tarp and 3 yards for the beak will run you about 40 bucks not including the grossgrain for the pullouts. Offset the tieouts for the beak so about 6 inches of material overlaps the poncho tarp and you end up with a big floorless tent.
The Equinox poncho tarp from Campmor (long) can be had for about the same cost and you just need to sew ridge line tieouts from webbing on it. Another option for you is to buy this tarp then just use mini binder clips to attach a sil-nylon door on the front. Just take three yards of sil nylon then sew a piece of paracord into the hem on one long side. The paracord makes a nice bead to fold the edge of the poncho tarp over and clip in place. You can trim the width of the door to suit your tastes and hem the edge.
Then set the poncho up Trapezoid style and you have a nice door to keep light rain or blowing rain off of you. Remember to set the back of the poncho tarp up to the weather. A big tyvek groundsheet makes it complete.
The poncho will be about 9 ounces and the door will be about 6 ounces depending on how wide you trim it. With the groundsheet you'll be around 20 ounces for shelter and raingear.