To throw a grenade in Starfield, follow these steps:

1. Open your Inventory.
2. Navigate to the "Throwables" section.
3. Select and equip the grenade or explosive you want to use (a small white tag will appear indicating it's equipped).
4. Return to the game.
5. Aim your crosshair where you want to throw, remembering the grenade follows an arc, so aim slightly higher than your target.
6. On PC, press the "G" key to throw the grenade.
7. On Xbox, press the "RB" button to throw.

Note that to see the grenade's trajectory arc while aiming, you need to unlock the Demolition Skill, which also enhances explosion radius and damage as you rank up the skill.