I don't believe there is a correct way as such but your Chair can ask those not voting if they are abstaining. It's is an irrelevant "vote" if you want to call it that as it doesn't register. For example if there are five people voting with one voting for and the other four abstaining, the vote in favour will count as a carried resolution. If you don't like what is being proposed, it is better to vote against rather than abstain.