CS:GO Nade Practice Commands can help you improve your grenade tossing skills by letting you see trajectory and speed in practice games. Here's how to enable CSGO Nade practice commands.
CSGO Nade Practice Commands: Hone Your Tosses
There are dozens of CSGO grenade practice guides all over the internet, ranging from simple commands to advanced configurations. Steam user ElchiBros has an easy to understand comprehensive guide on the CSGO community guides. You first have to create an offline game with bots. Head over to a map like Train or Overpass and turn off the bots. Make sure you can use console commands.
Punch in the following to see grenade trails: sv_cheats 1; sv_infinite_ammo 1; sv_grenade_trajectory 1; sv_grenade_trajectory_dash 1; sv_grenade_trajectory_thickness 1; mp_roundtime 60;mp_roundtime_defuse 60;mp_restartgame 1
You'll also get unlimited of each grenade. All you have to do is buy them, and then you can toss to your heart's content.
The trails are marked so that you can see when the grenade reaches each point in the trajectory. ElchiBros recommends you set the thickness of the trail and time for the trail to disappear longer for ease of use. You can input sv_grenade_trajectory_time "sec" to adjust the time.
If you don't want to manually input the commands, you can just download the configuration and plug that in. There are a few other notable guides with downloadable configs, like NadeKing's and Toby CS's. Practice throws on a variety of maps so you'll be ready for anything.