Every transaction you make on the marketplace to buy or sell an NFT can earn you votes. The way it works is:
If you buy an NFT on the marketplace - once your transaction is confirmed - you will actually get two NFTs. The one you wanted to buy, and a voting NFT (MINT). The voting NFT will contain your votes on it.
If you buy an item on the marketplace - your voting NFT will increase in votes based on the number of votes you earned.
Each vote has a current cost associated with it based on a bonding curve on the smart contract. So as more votes are sold/earned the price of each vote increases.
In order to earn votes - you need to buy an NFT for more than 2x the current voting cost. Since votes are split 50/50 between the buyer and seller - to earn 1 vote, you need to spend enough for at least 2 votes, so 1 goes to the buyer and 1 goes to the seller.
If you are not trading a super NFT - then the amount of votes is cut in half - or in other words - the super NFT's give you double the votes.
This means if your item isn't a super NFT - you need to spend 4x the current cost of a vote.
The current cost is 0.005 ETH per vote. Therefore, in order to gain 1 vote and voting NFT - you'd need to spend 4x that amount (2x if the contract is a super NFT).
So 0.02 ETH would earn you 1 vote at the price of 0.005.
You can see the current price of a vote at https://mintable.app/vote