To vote on a claim, click the four bars in the top-left-corner of the claim. This will open a panel of five ratings, from 0 to 4, that represent a spectrum of potential weight placed on a claim. Note that voting on a claim is a statement about that claim in context - contextually, how does it contribute to your view of its parent claim? - rather than a statement about that claim in isolation. To register your vote, click on your selected rating.
Kialo also has a guided voting system, which will systematically take you through claims you haven’t yet voted on. See: Guided Voting.