I build my rockets by building the main rocket first and saving it. I would give it a name, but the ones I use are extended to be launched by a multistage launcher. I designate them by Example, lets say the main rocket is called Main Rocket, the first one to be extended would be called MRMS-1, MR stands for Main Rocket, MS would be Multistage and the number at the back would tell me which extension model was it. If a extended model is modified and has a specific purpose, I would add a word at the end of the designation, for example, a model called FFMS-1 is equipped with lasers(using sunbeam laser mod) I would call it FFMS-1 Laser.