you should have done it whilst the suspension system was drained - it's a lot easier
but there is a "quick" way which doesn't involve depressurising the system, so here goes
1, remove the rear wheel arch trims (which involves removing the seat belt mounts and a few other bits
2, loosen the top bolt from the strut - that's the "clean" bits done the rest is under the car
3, with the car jacked up and all weight off the back wheels remove the wheels and the plastic guards from the suspension arms.
4, the base of the struts are held in with two bolts, loosen them but don't remove them
5, clamp the flexi hose that transfers the hydraulic fluid to the struts and loosen it half a turn - it may well be a tough one to turn as they can corrode in
6, remove the retaining bolts and pull the strut out - they come out a lot easier than they go back in but may still need a tap from the hammer
7, unwind the strut from the flexi hose and remove the copper compression washer (if you bought new ones for the spheres you should have a few spares as Merc package them in 20s
8, make sure you really clean any rust etc off the lower bracket and give it a coat of grease
to reassemble
1, compress the new strut about halfway and with a new copper washer fitted wind it onto the flexi hose union
2, now remove the hose clamp and try to wrestle the strut into place, it will either drop in really easily (if you get the length of the strut exactly right) or it won't quite fit, it's a bit trial and error but you need to at least part tighten the top bolt which is inside the car first
3, with the top of the strut secure and the hose in the correct place you can get the bottom bolts back in place and finish putting everything back together
4, reset the suspension system like you did when you changed the spheres.
Obviously this method is not the recommended procedure so the usual disclaimers apply - if it goes wrong it's not my fault
My advice on this job is only do it if the struts on your car are leaking, they either work or they don't - there is no halfway with them.
HTH
Andy