With the electronic ballot, once you have a membership number and a PIN, you can go and post some preferences and votes now, then return a day later, or a week later, or a month later, and change them, or add some more rankings. Your vote does not get counted until balloting closes.
The ballot is here: http://sasquan.org/hugo-awards/voting/
If you have not voted the Hugo Awards before, please note that it is an "Australian ballot," a preferential system whereby one ranks the nominees. You don't just vote for one. You can rank NO AWARD as if it were any other finalist; ahead of some nominees, behind others.
(Which is the way I believe one should use NO AWARD. As I have stated previously, I am opposed to the nuclear option of just blindly voting NO AWARD in every category).
Of course, you need to be a member to vote. Supporting Memberships will cost you $40. You can sign up to buy one at https://sasquan.swoc.us/sasquan/reg.php
In addition to voting privileges, a Supporting Membership will get you the convention's program book (usually a handsome item, though it varies from year to year) and other publications.
You can also sign up as an ATTENDING member and actually attend the convention, which is the course I strongly recommend for those who have the time and the money. Cons are fun, especially worldcon; that's what they are all about. Reading, panel discussions, the art show, the dealers' room, the masquerade, filksinging... all sorts of great stuff goes on. Something for all tastes. And EVERYONE is welcome, despite what you have heard. (Just don't be an asshole. Assholes get welcomed too, but the welcome wears out more quickly).
Both supporting and attending members get an electronic "Hugo packet" that will enable you to read many of the works nominated for this year's rockets. You should do that, no matter what side of the Puppy Wars you are on; we want informed voters. Yes, sadly, IMNSHO this is the weakest Hugo ballot in recent memory, thanks to the Puppy slates... but there's still some damn strong work there, especially in Novel and Dramatic Presentation. And of course it is possible that your own tastes may differ from mine.
So join, read, vote. And fifty years from now, when your fannish grandchildren ask you, "Say, gramps, what did you do in the Great Hugo War?" you'll have an answer for them.