The size of these minis can make easy conversions out of the question. For example I adh some 28 mm WW2 US army helemeted heads but they are way too small. 1/48 scale is too large.
I really wanted some more aliens in my rebellion, and really did not want the bald guy (A Bith I think??) in every unit. I found a Games Workshop beastman head that looked pretty close to a Devaronian.
I really love Twi'leks, especially after Hera in Rebels. For this conversion I shaved off the headgear down to a bald head shape.
I used Pro-create (A 2 part epoxy putty much like green stuff, only a bit less flexible and slightly less sticky but holds details well) I sculpted the Lekku, then once they had cured did the cloth head piece and ear covers.
Not a full professional job but I am happy with her and she adds some character to the unit.