Step 1: Research
One early decision I had to make was what version to make. The stargate from the movie has 2 distinct differences from the tv show version. I wanted my model to be of the movie version, but compatible enough to be a tv version so we had to make a few decisions on accuracy between the two. The movie version has an open top chevron and the the tv version is closed like all the rest. The movie version has engraved constellations, the tv version is raised. We went with exchangeable chevrons and engraved constellations.
Secondly, we chose one of the most common scales for model kits, 1/35th.
Step 2: Design
Once proportions were worked out I turned to a drawing program to work out all the details of the model and to break it down into parts.