How do put new rearend in?

Depends what you're replacing and why. If you're replacing the gears you take it to a mechanic with the special tools and training to properly adjust the bearing preload, pinion depth, backlash, and tooth contact pattern to prevent wear and noise. If you're replacing the entire axle assembly you unbolt the old one and bolt the new one in. Attach the parking brake cables and bleed the rear wheel cylinders.