When you say "the string will be spun around using electricity" do you mean the string itself will be spun on an arm or something, or do you mean that your airplane will be electric-powered?
"Blueprints" is a pretty strong word in this context, as a blueprint implies a completely thought-out design, where what you're building is going to be pretty experimental. First you need a working design. Then you need to draw it up.
Keep it light. If you're going to be flying these indoors out of the wind, as light as possible is best. You basically want an airplane that's really good at flying level or at a very slight climb, in a straight line, when you don't have the string on it. Then you should weight the outboard wing tip (the one you're not tying the string to) with about the weight of the string (that's heavy for us, but you're not doing stunts), tie the string on, and cross your fingers.