1) Jack the car fairly high under the crossmember.
2) Place jackstands under crossmember or rubber lift points behind wheels, and remove jack.
3) Drop the control arm as low as possible, and seat the top of the spring in its mount.
4) Compress the spring just enough to get it positioned in the groove on the arm. I have heard this can be done without compressing the spring at all, but I have not tried this personally. I used the cheap 'bolt type' compressors that are available from many places ($9.99), but the one you rented might work. If you do use the 'bolt type', you may be able to compress just the inside of the spring so that it bends towards the angle of the extended arm. This worked for me.
5) Once bottom of spring is seated, jack under control arm as close to balljoint as possible (this is important, since the weight of the car will not compress spring enough unless you are far enough out on the arm [leverage]).
Keep jacking until the weight of the car comes of the jack stand on that side, then you can install strut, etc....
Be careful. That spring has a tremendous amount of potential energy.
Hope that helps.
1986 300E 5-Speed 240k mi.