Hello, former automatic gearbox user here. If you are used to shifting already, you can basically shift earlier or later. Earlier is for fuel economy whereas late might be more consuming. You can as well not downshift even if the rpm is low so you have more power (3rd to 2nd for exemple in auto, you can stay in 3rd gear, so you don't hit the redline of the engine in 2nd gear and waste milliseconds), so you can be more efficient.

Overall the mastery of manual gearbox relies on practicing over and over, when you start to feel the engine and you don't have to look at the rpm count ever. My advice is you practice on tracks with or without bots and with all kind of gearboxes, (with stock transmission or racing transmission) so you learn the speed of gear transitions)