Install Qortal Core + UI, have your QORA address seed ready, launch the run.sh (GNU/Linux) or run.bat (Windoze) Script to start the core, run the UI, choose "Login", then "QORA Address Seed", enter your seed, done.

I did it in Windows and got constant >50% CPU usage by Qortal.exe. What's the catch?
Same Here... It's not even like one time thing.
It's constant... This needs to be investigated. I don't like my CPU tonbe burned-off without proper explaination.
I'm running Core on windows, and usage is under 10%. Its very old laptop (Acer from 2008). When Core is looking for nodes to connect, then it can be higher usage of CPU, but after connecting it should drop. Also, check version of Core you are running, if it is latest one. I can suggest you to join discord channel and describe your problems in details.