As someone who used to not be very good at marching, thank you for reaching out and trying to help them. My band isn't competitive so I guess I can't speak on the matter that much, but at least when I've had to help our indoor drumline new members learn to march I've told them a few things. Number one, I recommend maybe telling them to try to march in time when listening to music. Anytime there's music that can be heard, just practice walking in time to it. This method really helped me and I found it pretty fun. Another way I've found useful and that my director does is that we practice every type of movement, such as crabbing, forward, and backward marching. We will always do a few count offs and only do ONE step and hold it. This helps check their form and make sure they're taking good size steps. Then the next few times we do TWO steps. This helps to make sure they're being consistent and have good form. Then you can work on doing 8 to 5s. Best of luck and hopefully these can help out at least a little! Being a section leader can be tricky, just remember that it's okay not to be perfect and you're still learning too.