First you need to identify which drive is in the slim.
Lite on 4s = flash able but you need either a Sputnik probe, unlock, kamikaze hack, or unlocked tx pcb
Hitachi = negative
Lite on 5s = firmware in the works
Other than that you're all set.
Your all set. Grab a tutorial and boot up jungle flasher.
Read read read! Remember very step is crucial otherwise it wouldn't be there.