Practice from a sanchin stance with the intended standing leg forward. Bring the rear leg forward. Kicking knee/heel come in straight line toward target.
Hip rolls, aligning hip with target. Standing foot should be pointing in opposite direction of kick for balance. Kick extends toward target, thrusting and locking outward. Imagine feeling like a garden hose and water shoots outward into opponent upon impact. Retract kick and set down.
There are multiple ways to perform this kick. Find the one that works best for you. Options include a) lining up hip before lifting knee; b) bringing knee up as you roll hip, then extend; or c) kick rear foot straight to target (knee, hip, standing foot aligned), rolling the hip and extending just before the kick reaches its target.
Hand position is important. Do not drop hands. Hand on kicking side extends out and down across kicking leg. Back hand stays high near face to protect.
Point of impact is the heel on edge or bottom of foot.
Practice various ways of closing distance for kumite: 1) step behind from a side stance, lining up hip with target; 2) slide or long-stepping with front foot then kicking off rear foot as above; 3) bringing back leg next to/replacing front leg in side stance in a skipping motion (think Bill “Superfoot” Wallace).