You can use the ICO version of Jetton smart contract which is available here:
https://github.com/ton-blockchain/token-contract/blob/main/ft/jetton-minter-ICO.fc

You have to change the multiplier for your jetton. For example, multiplying to 1000, means that 1 TON equals 1000 of your jettons.

Then you can deploy it and anyone sending Toncoin to it, will receive your jettons.

Note that this is a template and you have to add additional functionality on top of it, like stopping it when it reaches a certain amount of jettons, or removing the admin address.