Let's have a quick dive.
1/3. Once you have the contract link or the contract address of the NFT you want to mint. Head over to the search bar and paste link or address in the search bar at the top of etherscan page.
Click on contract then write contract
2/3. Click on connect to web3. once you have done that.. next you find "mint" bar.. click on it to reveal a dropdown, there are usually two boxes to fill -
mint [ price of mint. // if its a free mint simply input 0. ]
3/3. Count payable (the number of NFTs you want to mint):
If you input more than you are allowed to, and you try to mint, you will receive a high gas estimate, approximately 1 ETH or more, which means there is an error.
BONUS - Once you have filled the boxes correctly, you will see a write button. Click on it and approve transaction in your wallet.