It depends on the algorithm you chose. For example, for Algo 1, operator 1's envelope would be your traditional VCA envelope. However, the operator envelopes for operators 2 and 3 would just effectively change the modulation index of the respective operator.
So if you wanted to make a pad using algorithm 1, set op 1's attack and release to be slow with a reasonable amount of sustain and use the envelopes of operators 2 and 3 to change the amount of FM over time, for example, by setting them to somewhat different values than operator 1's envelope.
I suggest using the Preen FM editor, it makes this very visual and you can see a bunch of the parameter at the same time.
Hope that helps.