One of the first things you have to learn as a new dancer, is foot positions. The general studio-trained western dance population, utilizes the numbered foot positions codified by the ballet tradition. During my years of teaching dance for colorguard, however, I have made some slight modifications to how I teach them.
Open first is achieved by placing the heels together with the toes apart. In colorguard, I teach first with the toes pointed at the 45's (90 degrees of turn-out between the feet). Because the competitive success of an ensemble depends on technical clarity, I focus on reinforcing this uniformity with a consistent turn-out across the ensemble. This also helps in cleaning choreography later, by training the group to understand where the 45 is. If you have a particularly well-trained or flexible ensemble, by all means, adjust your technique accordingly, but I find that this is an excellent place to start for beginning ensembles. Also be sure to reinforce good alignment through first. A neutral pelvis and stong, lifted core should be maintained, with turn-out initiated from the femoral head (the hip joint). Arches of the feet should remain lifted, and performers should feel a wide base through both feet as they make contact through the floor across the base of the toes and the heel. Watch that ankles do not roll out or in, but remain neutral (the feeling of lifted arches should assist in this).
The open second position that I teach for colorguard is slightly wider than the one you might use in a studio dance class. To emphasize both technical clarity across the ensemble, and reinforcing the colorguard ideal of "performing to the judges' box" (ie to an audience at a great distance), I ask my students to place their heels a little more than hips' distance apart. Another way of teaching this: if you tendu out to the side, and then place your heel where your toe touches the floor, this is "guard second." Again, be sure to maintain a neutral spine, strong core, neutral ankles, wide base through the feet, rotation from the hip....etc.
I almost never use Fifth position when teaching colorguard. Again, I want to provide my students with an achievable technique that can be consistent accross all flexibilities. In my experience, third provides a satisfactory "crossed" position of the feet for a guard context. Have your students place the heel of their front foot in the arch of their back foot. Toes should still point to the 45's, and hips should remain even and flat to the front. Again, remind the ensemble to turn out from the hips, maintain a neutral pelvis, stay supported through the center, neutral ankles....etc.
I usually don't teach Fourth position until I feel the group has mastered First, Second, and Third. I do, however, like to use it in choreography (it's great as a transitional step, and looks nice under tosses while still providing a stable/wide base. Seriously, watch an ensemble toss from Fourth sometime and tell me that doesn't immediately elevate the vocabulary!) To teach Fourth, use Third as a starting point and simply shift the front foot forward. The front heel should still be directly in front of the arch of the back foot, but with about a foot's-length between the two. All the same rules apply from third: turn-out from the hips with toes at the 45's, neutral pelvis, supported core, neutral ankles....pay particular attention to hip facings in this position! It is easy for performers to twist their pelvis so that the hips are no longer flat to the front. The weight should be centered directly between the two feet, hips should remain front, and rotation through the hip sockets should maintain that equal 45 degrees on each side.