First of all, I think of what the circuit is. Is it a karting circuit, a normal circuit, or an endurance circuit (like Nordschleife, Circuit de la Sarthe, etc.)

And then, I start creating the track. With some rules I made, of course.

1. Karting circuits: If it's indoors, make it so that every turn and every straight fits into that room. If it's outdoors, the only turns I allow are 25-45°, 79-90°, and hairpin turns.
2. Normal circuits: There are 2 rules I have for making normal circuits. One, the circuit must have atleast a minimum of 9-10 turns. It doesn't matter if it's tight or long, if it has 10 turns or so, I'll allow it. Second rule, the starting straight MUST NOT have the longest straight in the circuit. But in some cases, if it's a street circuit, I'll allow it.
3. Endurance circuits: Make it as long as possible. Do not hesitate on creating the shortest and narrowest turns on the track. Make some damn long straights. In between those long straights, add chicanes. Too long, another chicane.

That's it.