You basically just open a channel to their node. Some nodes have limitations, like a min amount needed. Once it's open you have a channel with outbound liquidity only in the beginning. As you spend you gain inbound liquidity as well. There's also websites where you can organize to make circles so you get for example 2 channels, one in and one out from another person. 

 And services you can pay for inbound liquidity. Basically you pay for them to open a channel to you. 

 Thanks! Can you link one of those websites you mentioned? I've heard of those circles of fire that people would make way back, and that would be interesting to try out. 

 For example https://lightningnetwork.plus/

Or you find 2 (or more) plebs on Nostr to join a circle