If you are indeed Mexican, the subjunctive shouldn’t apply anymore, as there is no doubt/speculation/hope involved. (Caveat: Non-native and also a bit rusty)

What you mean is "si fueras mexicano". "si seas" is just wrong.

"fueras/fueses" would be the right verb form, but that would be equivalent to "How to vote for Trump if you WERE Mexican" as opposed to "How to vote for Trump if you ARE Mexican".