Students rated 300 should know the following things before they come to the 400 group.
KNOW THESE STONE COLD!
P + P + P vs. P + P + P . Setup 3 white pawns on c4,d4,e4 versus 3 black pawns on c6,d6,e6 ~Learn how to break thru 100% of the time
Diagonals & the Bishop Setup a dozen or so black pieces at random. Use a white bishop to mop them up. Switch colors & repeat. Start naming diagonals, ?how many go thru the center?
Round the clock. From your head w/o looking at a board, call out every single edge square , in correct order. Then say them backwards.
What are the most typical mistakes?
Bad development.
Neglecting the King
Misjudging threats
Ignoring pins!
Premature aggresion
Miscalculation
Impulsivenes
Pawn snatching
Chess blindness or blunders whick can usually be due to carelessness