You can purchase kits for making quilts from either Thru Hiker or Ray-Way. The Thru Hiker kit is 800 fill down and comes with access to instructions; the Ray-Way kit is synthetic and comes with detailed instructions. I have made both as well as a home designed sleeping bag. I made a number of modifications to the Thru Hiker design including a head hole so the quilt can be worn as an insulated poncho.

Unless you already know how to sew, I would recommend you start with a simpler project than the Thru Hiker kit. On the other hand, I would recommend the Ray-Way kit to a rank novice. The reason that I am mentioning quilts is that there is not that big a step from making quilts to making sleeping bags. Moreover, you may find that a quilt suits your purposes just fine and forget about making a bag.

Also, if you log on to the Practical Backpacker forum, there are descriptions of one quilt project or another in the MYOG section. Here you can find out, in excruciating detail, the trials and tribulations of inexperienced sewers as they successfully make their own sleeping gear.