If you would like to know more the procedure on how to do a Flash Loan, check out the documentation.

Aave developers are also available in the official discord channel to help with the process.

A flashloan is borrowed and paid in the same transaction. You don't need any collateral for a flashloan. The point of borrowing and repaying in the same transacion is to take advantage of arbitrage opportunities that you don't have enough assets for. Furucombo allows you to create flashloan strategies without coding. However, keep in mind that there are a lot of bots that monitor the chain and they may steal this opportunity from you by copying your transaction and paying higher gas fee.