Alright, let's breakdown the focus & form of each movement!

Step-ups: work our legs & booty!

If you're using a chair, prop it against a wall so that it doesn't move & cause an accident!
Step your entire foot onto the chair/bench you’re using. Don’t let your heel hang off the edge.
Drive your heel down into the step as you stand - that way you’re engaging your glutes & not straining your knee.
Also, as you step up, focus on using the leg of your elevated foot to pull your body forward & upward instead of pushing off of your back foot.
The step upward isn't the only important part though! Controlling the step down will work your glutes & thighs double-time.

Tricep Dips: work the group of muscles on the backside of our arm!

Fingertips face your body
Bending your knees to flatten your feet on the ground will give your arms some assistance [& you can 100% do that even with the leg lifts]
The more you extend your feet forward, the more challenging it’ll be.
Keep your lats engaged & your neck long! [See how my neck hasn’t disappeared in this photo? Neither should yours!]
Hug your elbows toward one another so that they point back behind you as they bend. Avoid letting them bow out to the sides - not great for your wrists & elbows.. [ouch!]
Bend your elbows to 90-degrees with each rep. Humping the air is NOT a tricep dip - hahaha - [refer back to the 2nd & 3rd tips if you’re struggling with this!]

Plank Drill ["Walk the Plank"]: works our core, shoulders & chest!

Do your best to keep your pelvis squared up with the ground as you march down to your forearms & up to your hands [the less your hips twist, the more you're working your core]!
The wider you put your feet, the easier it is to keep your hips from twisting when you march.
Keep tabs on the position of your hips too: 1. if they're poking up toward the sky, that will move the tension out of your core & into your neck & shoulders; 2. if they're sagging toward the floor, that will move the tension out of your core & into your lower back.

That's all folks! Get ready to get sweaty!