If you can't kneel because your ankles are too tight then these toe and ankle stretches may help. The key in all of them is to activate (and then relax) the toes by pressing them into the floor. I've also include simple movements that help you to add weight to the stretch and then take it off. In all cases work slowly and smoothly. Smoothly add weight and smoothly subtract it again.
To learn to kneel comfortably as shown below, start of on all fours.
Slowly move your hips and torso backwards . When your weight is off of your hands you can lift your hands. As you move your hips back bring your torso slowly upright. Press your toes down into the floor as you do so. Stop when you need to and slowly and smoothly return to the starting position. Repeat a few times (x3 or more) seeing if you can go a little bit further each time.
If your knees hurt when you do this, then work at sitting up slowly, and stop before the onset of knee pain.
Remember to press your toes into the floor as you sit back. Relax your toes as you go forwards again.
Try to push the tops of all ten toes evenly into the floor.
You will probably need several weeks (or months) of practice before you can knee comfortably. Once you can kneel comfortably you can increase the stretch by leaning back and placing your hands on the floor.
Slowly lift your knees to stretch the fronts of the ankles and the toes, then lower the knees.
Move slowly. Stop if you need to.
A stretch for the bottoms of the toes is to kneel with the toes tucked under.
Bend forwards initially with hands on the floor. This is the relaxed or resting position. Slowly sit up while pressing your toes into the floor. Then slowly bend forwards again and relax your toes.
Once you can get your butt to your heels, hold the seated position. Slowly press your toes down and then relax them.
Lengthen your spine as you press your toes down. Then relax your spine as you relax your toes.
To increase the toe stretch your can try walking your knees a little bit forwards (away from your toes.)
To stretch the backs of the ankles you can use the downward facing dog yoga pose. First try lifting your heels as high as possible. Then to help touch your heels to the floor (and stretch your ankles) focus on pulling upwards on the fronts of your feet.
You may also find this ankle stretching video on youtube helpful.
As a yoga teacher, I'm constantly exploring new exercises, new ways of doing yoga poses.
There is no single "right way" of doing a yoga pose. Instead, there are options. And the better you are at "feeling" your body, the better you can get at choosing the right option for your body as it is now.
For any technique, the point of practice is to learn feel it and to control it, so that it can be used without thinking about how to use it.
And that is more or less the approach taken in all of my ebooks and videos. They help you to feel your body and control it so that you can work towards using it effectively in anything that you do.