Make sure you fast fall after the nair connects rather than before; a lot of people get stuck in the habit of fast falling at the very apex of their jump rather than understanding that during the basic SHFFL practice you're actually ffing as the strong hit of nair is coming out and that being what you need to focus on rather than getting in to the habit of relying on a specific timing to FF.
Usually you wanna hit anywhere on the lower half of the shield; this way you can shine -> grab, double shine, or waveshine back all to beat buffered oos options. Waveshine back (if you spaced shine correctly) dodges grab, roll in, lets you react to full hops, lets you break away to laser, you can do a drill after the waveshine to beat later options like a delayed wd oos. Waveshine back is a pretty non-committal move so you mostly want to use it when you've got the stage control advantage
Double shine catches grab and people dropping their shield earlier, shine grab is usually the go to until they start rolling, at least if you're hell-bent on using shine in your pressure.
Usually when you get jabbed or grabbed between the nair and shine it means you didn't ff correctly or your nair was too high. Medium height nairs are ok if you can FF them properly, although medium height nairs make it to where you basically have to shine afterward rather than dash back, roll or another aerial, so it limits your options a tad bit.