finishing ERC20 token smart contract functions in solidity

I'm Doing a personal proyect and i started doing a smart contract and i have most of the basic functions working ( transfer,minting,burning,allowance,freeze account), but i am having trouble making the buy function where you can sell the token for an X amount of ether. So im looking for help to make the buy/sell function. It's a test proyect to deploy in the test network and try it out. I'm currently using truffle ganache to do all the testing