There's no right way to do this, but here's my method for what it's worth:
(I'm hoping to start querying when my novel is fully polished, probably about a week from now, so I'm compiling names now.)
1. I use query tracker and I pay the $25/year for the pro. It's not much and you get extra data and can save multiple projects at time, but the actual list is free. (Side note--recently had a run in with their costumer service after I made a mistake and double paid. They fixed it within 24 hours and were super nice about it.)
2. I follow and periodically go through #MSWL (with the MSWL blog in one tab, QT in the other) and save agents who fit my genre to my to query list.
3. When I've gathered a good batch of names, I google them and look them up on Preditors and Editors and Absolute Write and delete them if there's any red flags.
4. I make notes on each agent about the vibe I get from them on social media and with interviews, and use those notes to loosely rank them.
5. Query in batches of five.
I have yet to get an offer, much less competing offers, but I have a list of questions ready to ask in case of the CALL to help me choose when/if.