The first 6 strides.
The number of strides taken during acceleration varies for each person, but it is important to identify how many strides will be taken in acceleration so they you can be consistent. Start by trying this with the first 6 strides. Rather than thinking of 6 strides, simplify it by thinking of 3 strides with the ride leg (or left leg). Focusing on one side gives you time to think about and execute a full range of motion rather than switching between what the left leg is doing and what the right leg is doing. The first few strides are very important because they determine how fast you can reach your top speed, and they also establish the mechanics of ground contact, pulling on the ground, and scissoring.
Two important components of the leg action are driving the lead knee and scissoring the legs. Driving the knee is just rotating the thigh, but to call it a “high knee” seems like it may mislead some people to think of a vertical motion rather than a forward drive. If you lift your knee as high as you can while standing upright, this is the same knee position that you want with you body near a 45 degree forward body angle during acceleration.
A good place to start in developing these components is to watch video of yourself. If your trail leg is not fully extended, especially in the first stride, then focus on a faster and longer knee drive with the opposite leg. The legs will do the opposite of each other. If the knee is passive, then the trail leg will probably also be passive. A longer knee drive gives the trail leg more time to extend. The knee drive also stretches the glutes allowing them to accelerate the thigh over a longer range of motion.
Another problem to identify in the video is a curling trail leg. This can be corrected with the knee-drive combined with scissoring the legs. If you only focus on the knee drive, then the scissoring may be passive and the trail leg has time to curl. By simultaneously activating the glutes of the lead leg and the hip flexor of the trail leg, the legs will scissor. The trail leg will move forward rather than delaying its forward movement by curling.
A related problem to identify is the foot contacting the ground while the trailing knee is not aligned with it (or nearly aligned with it). This may be a result of passive scissoring, and/or letting the lead leg straighten too much before contact. A straightening leg increases the angle (an angle closer to vertical) that the lower leg makes with the ground. This allows the contact point to be further ahead and away from the trailing knee.
A final problem to identify is the torso coming up too fast. To correct this, keep your abs crunched curving your torso forward and look at the ground. Set this position before you start and maintain it through the first 6 steps. Learn this strict movement then adjust to what is appropriate for you and for your sport.