Call your local police department. Usually, there's an officer trained in the correct installation of child safety seats. Sometimes, they even give out free car seats for a small donation to the program.
First, get a very thick beach towel and double/triple it up. Lay it on the seat where you're going to put the car seat. The pressure of having the car seat straped in over time, will leave indentations in your leather. Lesson learned.
When the police installed my car seat, they used 3 pieces of a pool noodle to level it out... works great. The seatbelt was strung through the car seat and latched. The trick is to leave the seatbelt short of the latch and let it retract some... then bear your weight onto the car seat to latch the belt in place. I'm talking full body weight on the car seat so that you can barely get the belt to click in. When you release your weight from the car seat, it's very very stable. If the carseat moves side to side at all, it is improperly installed.