Don't buy liquid flux. Makes a right mess and it gets contaminated quickly. I use a flux pen. One of these:
https://uk.rs-online.com/web/p/solde...aners/3217289/
If you go for a no clean flux pen, you don't have to clean it off, but it gets a little sticky so it's worth cleaning it off anyway. I use the above with standard 99.9% isopropyl alcohol applied with a natural hair paint brush (nicked from the wife) then wash that off with distilled water. You need to do that wash process 3 times normally to get rid of the sticky.
If you go for a flux pen which isn't no clean, you have to clean it off with solvents properly or it'll cause damage in time.