How can you make my 1990 Chevy cavalier front wheel drive into a full time posi traction end?

You can't make a 1990 Chevy Cavalier front-wheel drive into a full-time posi-traction rear-wheel drive car. Posi-traction (or limited-slip differential) is a feature of rear-wheel drive vehicles. The Cavalier is front-wheel drive; it has no rear differential to modify. The differential is integral to the transmission in a front-wheel drive car.

To get posi-traction, you'd need a completely different car—a rear-wheel-drive vehicle with a differential capable of being modified or replaced with a limited-slip differential. Even then, converting to a "full-time" posi-traction would require significant modification and likely isn't practical or cost-effective for most applications. A standard limited-slip differential is the typical goal.