The Grind game mode was unlocked for me after I completed day 1 of The Job after defusing a bomb.You must defuse all 10 in one game as this isn't cumulative over multiple games
The Grind is an endless mode of bomb disarming, luckily you can pause the game at any time using
to go through the rules as slowly as you like. I also found it helpful to make a drawing I could write on and go through with the rules. Id go through the rules in order (1-4) and retrace my steps if a variable changed like if page 4 told me page 2 had something wrong on it id go back.
Here's a picture of my notes for example, its a bit messy
Every 3 bombs is a filing bomb where you order names by first/last A-Z/Z-A if you did that in day 2 of The Job, I personally found those easier than the bombs but if you want to avoid those then do The Grind ASAP to avoid having your unlocked game elements in it. The filling still counts as a bomb for the 10.