Step-by-Step Guide:

Visit the Overlay testnet page.
Connect your wallet.
Change the network to Arbitrum Sepolia.
Join their Discord channel and get some testnet ETH and OV from the #claim-ovl channel.
Now go back to the Overlay testnet page and make trades.
Also, complete their Galxe quest.
A total of 1.5% of the total OV supply will be airdropped to testnet participants.
They’re also airdropping ex-OVL holders, PlanckCat DAO NFT Holders, Overlay Hoodie buyers, and Overlay NFT holders. You can check the eligibility from here.
For more information regarding the testnet airdrop, read this article.