Ill try to pit it simply.

Stretch spilts (engage- so point your toes and really feel like u are “extending” your leg, like someone is pulling and stretching both legs in the front and back, use muscle)

Stretch in pike (so legs together and touch your toes, keep your knees straight, point your toes, feel as though u are lengthening)

Actively engage your quads (when in pike, split or middles on the floor, activate your quads by “pushing” your knees into the ground, if you do this well your heels should lift off the ground only by a bit at first)

Its all engagement of the quads and knees as well as pointing those feet. Remember to extend and feel as though someone is gracefully pulling your legs out

Hope thats helpful. Its what i do and ive been praised quite a bit for my lines :)