According to pvolve.com "Using the slant board places more pressure on the feet than standing on a flat surface. This makes it a great tool to turn on those muscles in the feet and condition them in ways that they may not get challenged in every day walking but are important in things like sports or dance—anything that requires a lot of footwork. Trust me, the feet do much more than walking; strengthening them and establishing better biomechanics in the foot will, in turn, enable your thighs and glutes to work properly, as well."
StrongTek the maker of the full coverage slant board that I recommend provides the following advice: "your slant board has 4 customizable settings with varying angles. As a general rule, it’s best to start off by using the mildest incline to begin getting acquainted with the board itself.
Now that you’ve got the board set-up in the desired angle, simply place it flat on the ground and step onto it [with both feet], ensuring that your feet are firmly placed on the bottom of the base.
If you find that you’re having a difficult time staying balanced, feel free to reposition the board next to a couch or handrail so you have something to brace yourself against. The key to this exercise is being able to focus entirely on the stretching of the muscles, not your balance.
Find yourself in a comfortable, upright position. Take note of your breathing and focus on becoming incredibly relaxed.
Once you’ve become familiar with the slant board, there are plenty of other opportunities to level up and get an even deeper stretch.
Onto a slightly more rigorous way to use your slant board.
Choose your desired slant level [start with the lowest setting to begin with] and get on the board just as you did with the previous exercise, facing uphill on the slant.
Now we’re ready to take things up another notch."
Remember to keep your knees slightly bent and your upper body in an upright position.
Find something to focus your attention on about 12 feet away from you. It can be a spot on the floor a picture on a wall, whatever works to hold your focus.
Now, with your eyes on the focal point you have chosen, raise your hands up over your head and then lift one leg as high as comfortable, but no higher than your waist, and balance yourself on the opposite leg. Try holding this position for a couple of seconds at first and then increase the amount of time as you feel steadier.
As mentioned above, when you are first starting out or increasing the angle of the slant board, feel free to position the board next to a couch, a chair, a wall, handrail or anything else that can support you and keep you steady. Eventually you will need this support less and less, until not at all. Challenge yourself, but don't push yourself so that you are scared, uncomfortable or in danger of falling. You need to stay as relaxed as possible.
Once done with the one leg, do the same with the opposite one.
Now let's change how your body is inclined by doing the same exercise, one leg at a time, but facing downhill, and then sideways on the slant board. You will be balancing six times, each leg in three different alignments.
As you get better at balancing you'll no longer need the outside support and you'll steadily balance for longer and longer. Once you can comfortably balance for at least 20 seconds, increase the incline and start again.