Craft fire arrows by selecting the bow (keyboard 1 or D-Pad Up) then without aiming, hold the Alternate Fire button (Middle Mouse/R1/RB). Lara then combines a regular arrow, cloth, and fat/oil from inventory to make a fire arrow.
To use it, hold Aim (Right Mouse/Left Trigger), aim with the mouse or right stick, then hold Alt Fire (Middle Mouse/R1/RB) briefly...
...and release to shoot.
Grab one of the empty tin cans and hold Alt Fire (Middle Mouse/R1/RB) to craft a shrapnel grenade using salvage from your inventory. As you hold the button, you'll see the can-grenade icon fill while Lara works. When it's full, the grenade is ready.
Hold Aim (Right Mouse/Left Trigger) and you'll see a white arc. Adjust with the mouse or right stick, then press Left Mouse/Right Trigger to throw the can.
The explosion should take out a weak enemy like one of these guys.
If you change your mind, you can hold Duck/Drop briefly to drop the empty can or the crafted grenade without throwing it.
Molotov cocktails work the same way. Grab an empty bottle.
Hold Alt Fire (Middle Mouse/R1/RB) to add cloth from your inventory and light it. As Lara works, you'll see the bottle icon fill. When it's full, the Molotov is ready.
Hold Aim (Right Mouse/Left Trigger) and direct the white arc with the mouse or right stick.
Then press Left Mouse/Right Trigger to throw the flaming bottle. When it smashes, the fire spreads to anyone standing nearby.