Don't worry if you don't know how to unlock him, it's really simple

You must collect
• 4 candles
  
• 1 sammy mask

Too find these you need to search through the toilets and lockers you will find around the levels.

Once you get all the things that you need, it's time to do the ritual.
You must go into the guys bathroom

And the ritual will be here, just interact with it.
And now you have unlocked him.

To play as him you must change your character in this place

Now, you might realise that you didn't actually change characters, to see this happen, you must enter a level.


Gameplay basics of Sam Law


I'm gonna be honest, he is way harder to play compared to Boris, he can't run, and to compensate he can only go thru ink portals like this one:
(it's the wall that is to the side of the bacon soup dispenser.)

You will encounter this around the levels, it is important to search for them when you enter level since, they can save your life, you must also remember where they are located, and
to camp those areas.
You should also not put yourself in to much risks.
The Projectionist, and the Alice Angel levels must be hardest one, since Alice is really fast, and the Projectionist levels have a lack of ink portals.
You must only be spotted by the monsters if you have a ink portal or hiding place nearby.

And that's everything! And remember, you must practice, if you don't, you'll never develope your own tactics.