Curl-up

Lie flat on your back with your knees bent and feet flat on the ground.

Place your hands behind your head with your elbows pointing outwards.

Engaging your abs, slowly lift your upper body off the ground, curling forward until your torso is at a 45-degree angle.

Pause for a moment at the top, then slowly lower your upper body back down to the starting position.

Repeat for the desired number of repetitions.

Recruiting larger muscle groups to "help" move the weight instead of isolating Abs — swinging momentum or shifting the torso takes tension off the muscle this exercise is meant to train.

Rounding the upper back or shrugging the shoulders up toward the ears at the start of the pull instead of initiating with the target muscle.

Rushing through reps using momentum rather than controlling both the lifting and lowering phase, which is especially important since there's no external load to force good tempo.

Skipping a warm-up set at lighter load/intensity — even "beginner" exercises benefit from a rehearsal set to groove the movement pattern before working sets.