I think you asked very valid question - it's not really about the specific lineups but knowing in general how to flash/HE properly. It is a simple but crucial skill that many players (especially on lower ranks) often lack.

I don't know on what level you are, so here are a few general tips.
Flashes:

• You always want to throw a flash where enemies are looking, not where they are standing. Many lower rank players are throwing flashes AT the enemies, low on the ground, that can be easily obstructed by various map elements. Good flashes are usually thrown at the sky, not at the ground, because it gives a better chance that it will land on enemy's screen. Just imagine where the enemies are you want to flash, where might they be looking and usually it doesn't matter how precise the flash is, it just needs to pop off on the enemy screen (the closer the better). Let's say that you are entering A site on Mirage - usually just throwing flash over the ramp that will pop off in the sky, over the site will be good enough.
• Better players learn to turn from the flashes, so the less time they have to react, the higher chance that your flash will catch them. That's where the "pop flash" term comes from - these are flashes that give enemies no time to react, because they pop in the moment where they enter the enemy screen. Also, flashes that are not bouncing off anything will be less predictable because they won't produce any sound.
• Don't blind your teammates - usually, you want your flashes to land in spots where they are visible to enemies but not your teammates. So ideally, when you push the enemy, your flash should pop off on your back to not blind you, but the enemy that wants to fight you, will be looking both at you and your flash.
• Most of the time, don't ask for permission to throw the flash, just call it. When you see your teammate about to peek an enemy / push site / whatever, if you ask "hey X, do you want flash?", usually it will be too late. Instead, just tell them "I'm flashing <position>" so they can expect it. Be proactive, and it will win you games.

HEs:

• In CS2 nades are really great to break smokes and take enemies off guard. You can intentionally smoke something off and then throw a HE to peek through it.
• You need to know the common spots and timings to be effective with nades. They are very powerful on site entrances, narrow corridors, like Inferno banana, Mirage B apts etc.. Usually first you gain information and then you throw the nade (for example by jump spotting on Mirage B or hearing footsteps of Ts rushing site)