I would think that you loosen the mount bolts first, then the x-member.
After that you'd need to drop it and take the top mount bolt off.
Replace mount, tighten top mount bolt, then lift it and tighten the x-member bolts, and finally the 2 lower mount bolts.

However, the way you tighten it etc shouldn't effect your idle.
And, are you sure it's your transmission mount and not your engine mounts?

Also, did you find out if the transmission mount bolts should have threadlock?

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Smoke Silver 1994 E220 Coupe Automatic
0 kilometers 6 September 1994
100,635 kilometers - 6 September 2010