Everything other than the mode/entwine choice (and paying the , of course) is done on resolution.

Assuming the entwine cost has been paid, the effect is (1) sacrifice any number of creatures, (2) gain 3 life for each sacrificed creature, (3) choose X, (4) pay X life, and (5) look at the top X cards of your library, put one into your hand, and exile the rest.