Depends on how your node is set up. With Alby Hub this is integrated easily under the node management page. You can also purchase directly from the LSP if you don't have something like Alby's build in integration. You could take a look at the liquidity provider "lnserver". They have tutorials walking you through the process on different node setups.
Discussion
Also I have just had a look and on chain transaction fees are quite high right now so that may make it expensive. It's better to open channels when the fees are low.