Simple Flashloan Tutorial - https://www.youtube.com/watch?v=2lY3sbjQBwk

flashloan.test.js

// Right click on the script name and hit "Run" to execute
const { expect } = require("chai");
const { ethers } = require("hardhat");


// Mainnet DAI Address
const DAI = "0x8f3Cf7ad23Cd3CaDbD9735AFf958023239c6A063";
// Random user's address that happens to have a lot of DAI on Polygon Mainnet
const DAI_WHALE = "0xdfD74E3752c187c4BA899756238C76cbEEfa954B";

// Mainnet Pool contract address
const POOL_ADDRESS_PROVIDER = "0xa97684ead0e402dc232d5a977953df7ecbab3cdb";

describe("Storage", function () {
  it("test initial value", async function () {
    const SimpleFlashLoan = await ethers.getContractFactory("SimpleFlashLoan");
    let simpleFlashLoan = SimpleFlashLoan.attach("0x45C823850AeB23E2ed11c9105d5f666C0BD539d1")

    const token = await ethers.getContractAt("IERC20", DAI);
    const BALANCE_AMOUNT_DAI = ethers.utils.parseEther("2000");

    const tx = await simpleFlashLoan.createFlashLoan(DAI, ethers.utils.parseEther("1000")); // Borrow 1000 DAI in a Flash Loan with no upfront collateral
    await tx.wait();
    const remainingBalance = await token.balanceOf(simpleFlashLoan.address); // Check the balance of DAI in the Flash Loan contract afterwards
    console.log(`remaining balance: ${remainingBalance}`)
    expect(remainingBalance.lt(BALANCE_AMOUNT_DAI)).to.be.true;
  });
});

SimpleFlashLoan.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.4;

import "@openzeppelin/contracts/utils/math/SafeMath.sol";
import "@aave/core-v3/contracts/flashloan/base/FlashLoanSimpleReceiverBase.sol";
import "@openzeppelin/contracts/token/ERC20/IERC20.sol";


contract SimpleFlashLoan is FlashLoanSimpleReceiverBase {
  using SafeMath for uint;
  event Log(address asset, uint val);

  constructor(IPoolAddressesProvider provider) FlashLoanSimpleReceiverBase(provider) {}

  function createFlashLoan(address asset, uint amount) external {
    address receiver = address(this);
    bytes memory params = "";
    uint16 referralCode = 0;

    emit Log(asset, IERC20(asset).balanceOf(address(this)));

    POOL.flashLoanSimple(
      receiver,
      asset,
      amount,
      params,
      referralCode
    );
  }

  function executeOperation(
    address asset,
    uint256 amount,
    uint256 premium,
    address initiator,
    bytes calldata params
  ) external returns (bool){
    // run arbitrage or liquidations here
    // abi.decode(params) to decode params

    emit Log(asset, IERC20(asset).balanceOf(address(this)));
    
    uint amountOwing = amount.add(premium);
    IERC20(asset).approve(address(POOL), amountOwing);

    return true;
  }
}

StartHardhatNode

$ npx hardhat init
$ npx hardhat node --fork https://polygon-rpc.com/      # replace https://polygon-rpc.com/ with YOUR_RPC_URL

$ npx hardhat console --network local # to connect with node via JS

transferDai.js

// this transfer DAI to your account

const DAI = "0x8f3Cf7ad23Cd3CaDbD9735AFf958023239c6A063";
// Random user's address that happens to have a lot of DAI on Polygon Mainnet
const DAI_WHALE = "0x25E53Fe97360906cb990417cf0292a25DcF06075";

const token = await ethers.getContractAt("IERC20", DAI);
const BALANCE_AMOUNT_DAI = ethers.utils.parseEther("2000");


await hre.network.provider.request({
      method: "hardhat_impersonateAccount",
      params: [DAI_WHALE],
    });
const signer = await ethers.getSigner(DAI_WHALE);

await token.connect(signer).transfer("ENTER ADDRESS HERE", BALANCE_AMOUNT_DAI);