This is a quick guide to footwork and range control. I'm sure I could go into greater detail, but this covers most of the basics. Feel free to comment if I've missed something.
Footwork is a difficult topic to teach, especially to new fighters. They have far more pressing matters on their mind, like keeping a good guard and swinging. Most new guys, therefore, end up with a quick and dirty intro to footwork that amounts to mostly how to position their feet in static defense. This ends up being a lesson in stance, more than footwork.
Proper footwork is far more about dynamic scenarios, ie. how to move and position yourself and your opponent to gain an advantage in the fight. Any time you advance or withdraw from the enemy, you are using footwork to engage in the "range game". By shifting around your opponent along a diagonal, you change the angle of both your defense and theirs. These two concepts combine to make footwork one of the most essential keys to both attack and defense.
Most lessons start with the basics of stance, positioning your feet about shoulder width apart with one foot more forward than the other. The goal is to give you a good, solid base while angling much of your body away from the enemy, reducing the area they can target. Which foot is forward is more of a matter of preference and situation. Shield foot forward, combined with good shield work, can prove extremely good at fighting on the defensive from multiple angles. It draws your shield to cover more of your sword side and can be particularly effective at cutting off certain angles to protect against spears. Sword foot forward offers a little faster attack because your sword will be closer to the target, but requires greater skill in sword blocking and counters to be at its most effective. This is why sword foot forward is often used by a majority of experienced fighters for most situations.
Now, of course, shifting your weight to your rear leg allows you to move your front leg quickly. This is usually advised when fighting in a more static situation, like in the middle of a line fight. However, as we begin to move and shift in a more dynamic situation, it is far better to keep your weight more evenly distributed. You need to be able to shift directions quickly. Each time you place too much weight on one foot or another, you lock it in place for a moment. Skilled opponents will wait for you to plant your foot before they swing at your leg, making it nearly impossible for you to dodge.
When advancing in the opponents range, slide in your current stance unless you have a reason to change position. If you step normally, you will end up changing the angle of your defense and planting one foot as you walk. Changing your stance while in the opponents range has a similar effect. If your opponent's sword is free, they'll likely be able to take your leg with ease in either case.
"The range game" is a pretty broad topic, but there are three main ranges you should concern yourself with when it comes to footwork. The worst range to be in is when your opponent is the only one close enough to swing. This makes you extremely vulnerable. Ideally, you need to either be out of their range or in your own range. Note, that the if you can swing, your opponent may also be in range.
While approaching the enemy, try to maintain your defenses until you have something to swing at. There is little use in having solid footwork if you completely forget to keep your shield up. If you have a shorter weapon than your opponent, you will have to weather a few swings on your way in.
Now, if you are trying to maintain a certain range, you need to match and counter your opponent in footwork in order to keep them where you want them to be. If they step towards you to reduce range to their preference, slide back. If they try to retreat out of your preferred range, slide up. Again, maintain your defense either way.
Most people learn to advance and retreat in line with their opponent. By adding in angles to the mix, you can gain even greater advantage in range. If you step back and to the side away from their sword, you drastically increase the range from their weapon with the same amount of motion. On the advance, you can advance towards their shield side, angling past them, in order to keep your shield in the way of most of their attacks. Note that two fighters with the same handedness cause angles to offer the same benefits and drawbacks to the other fighter. If you advance towards the opponent's sword arm, you open up an angle for both of you to attack.
When it comes to targeting different areas, footwork can change what is open to you. Bending at the knees lowers your arm, bringing your sword closer to low targets. Stepping and bending your knees can change from being completely out of range to being able to target a leg easily. Much the same can be said for advancing at an angle, bringing you closer to strike at an arm.
One trick some veteran fighters use is to advance in a spiral, slowly closing the range as they circle. This allows them to sometimes catch an opponent off guard when they fail to notice that the range has changed. This is most commonly used in a one on one fight or when an experienced fighter is outnumbered.
When pushing in on an opponent, part of your goal is to force them to take a static defense. If you can manage to get their feet planted, their legs become easy targets and they have little they can do to out maneuver you. Part of how I manage to do this is to change the angle of my advance, starting with one side, then changing to push past them on the other side. This does leave one of my legs somewhat vulnerable as I have to shift my weight from one direction to the other, but most opponents will focus on keeping their defenses since I am coming into range to swing.
When shifting to the opposite angle, the opponent is likely to have planted their defense to the first angle, and now have to shift their weight to rotate the opposite way. This makes it very difficult for them to get their feet and torso repositioned. As I shift to the new direction, I often change stance such that my outside leg is the lead leg. This forces my defenses to angle towards the opponent as I move past.