To unlock Lethal difficulty the old fashioned way you will need to finish all 18 campaign checkpoints on Hard difficulty.
Level Select will show what difficulty you’ve done each checkpoint on.
However you can have a friend invite you to a Lethal campaign if they’ve already unlocked it and all your progress will count towards Overstrike Elite.
If the host drops out the new host will continue on Lethal difficulty.
Even if you stop you can go to Level Select and choose any checkpoint and play on Lethal difficulty.
DO NOT SWITCH THE DIFFICULTY, otherwise you will be unable to reselect it.
Ideally you should play a few games of Echelon first to level up and obtain some decent team perks as well as familiarizing yourself with the enemy types seen through the campaign.
Ideal Team Perks: Vitality, Last Gasp, Quickdraw.
The only major difference on Lethal, apart from harder enemies and going down easier is that team mates will be unable to fight out of a Spec Ops’ and Infiltrator’s struggle QTEs. They MUST be saved.