I make all my own patterns for my stained glass designs and I will divulge a few tips that have helped me. First of all I use butcher paper for my renderings because it comes in a large quantity and is fairly sturdy. You can get rolls of butcher paper in warehouse outlets like Costco and Smart and Final.
Once I have an idea I will use pencil and start to draw. The above picture is an idea that came to me while I was practicing yoga at the Bikram (HOT yoga) studio! I get lots of my ideas while doing yoga (although I am supposed to be clearing my mind and staying present)!
When making a design for glass you have to consider where your cuts will be and if the cuts are feasible. In the above design the background is divided into four sections. After I drew the design, I realized that the middle section was too narrow for cutting and would have to be adjusted so I re-worked the lines and made them farther apart. I like to have the least number of cuts possible and keep it simple. I also try to make the cuts a natural part of the design as in this one, the cuts will be the branches of the plant. The leaves on this design will be made of wire and added afterwards so there are actually only five peices to cut (to be continued).