Should I focus on having the pressure on the back or front side of the foot when going down a slope? I find myself having it on the back of the foot (Used to this from squatting at the gym). Should it be spread evenly? What about the left and right side of the foot?
The most correct answer is that your weight distribution will dynamically change depending on the terrain, conditions, desired turn shape, your ski equipment, and even vary within a single turn. Learning how your skis react to changes in both fore-aft and lateral balance and pressure---and subsequently learning how to use these responses for increased performance---is a key skill and frustration in skiing. In order to keep things from straying too far, let's focus our discussion on a typical parallel turn by an intermediate skier on a nice groomed slope.
The first weight distribution to discuss is that the outside ski should have most of the weight/pressure (this is your right ski in a left turn and your left ski in a right turn). The goal would be ~80--90% of your weight on this outside ski in the middle of a turn, with a smooth shift to the opposite ski as you transition into the next turn.
Moving down the leg, our next cue is to generally have your shins pressing into the tongue of your boot. In addition to helping support your weight through the forces of ski turns, this allows you to effectively pressure the skis using leverage. The amount of force will vary over the turn, ramping up in the middle/apex of a turn and lessening as you exit the turn and transition into the next one. The correct amount of shin pressure will also greatly depend on the speeds and performance involved. For beginner/intermediate turns, think of "sagging" or "relaxing" into the tongues of your boots. For aggressive high-performance carving turns, you might be trying to fold your boots in half in the middle of a turn.
Finally, we move to weight distribution within your foot (the main focus of the OP question). Ski instructors commonly use the analogy of thinking of each foot as a tripod consisting of the heel, the base of your big toe, and the base of your pinky toe. In order for a tripod to not fall over, there needs to be some weight on each of these points. We will redistribute the pressure among these points in various points of a turn, remembering to keep some pressure on each of them. Through all of this discussion, we are trying to stay balanced on our skis. If we visualize a surfer, they have their entire foot planted on the surfboard---not up on their toes or back on their heels
First, lateral distribution (base of big toe vs base of pinky toe). In the middle of a turn, your outside foot will have pressure shifted from the pinky to the big toe and vice versa for your inside foot, i.e., pressure shifted towards the ski edge in the snow. As we transition to the next turn, we want to smoothly shift our pinky/big toe pressure to correspond to the new inside/outside foot. This weight distribution is usually fairly automatic as we shift our weight between inside and outside legs and tip our skis on edge.
Next, fore-aft distribution (heel vs pinky and big toe). This shifting is much more subtle and generally stays closer to an even split with a bias towards the front. We should (almost) never feel our heels lifting nor feel like we could raise our toes. When initiating a turn, there is a slight shift forward, which is carried through the apex, and then subtly shifted backwards as we exit a turn. Another way to visualize this is that we have our weight shifted slightly forwards when we want to hang onto our current set of edges and shifted ever-so-slightly rearwards when we want to switch edges. As a general rule, most beginning skiers have their weight too far back (a la a back squat), with ski instructors the world over saying to get that weight forward. To continue the weightlifting metaphor, the correct basic fore-aft distribution is more like the bottom of a deadlift. Also note that our ski posture is around perpendicular to the slope: on flat terrain, this means standing vertically; on a steeper slope, this means projecting ourselves forward and downhill as we go through the fall line.
A good drill for feeling this fore-aft distribution is side-slip on a groomed run. As you're side-slipping, play around with your fore-aft pressure distribution on your feet (and correspondingly move your hips/center-of-mass forwards and backwards). Get a feel for how the ski reacts and moves you as your shift your weight. Master this feeling---you should be able to side-slip directly downhill, forwards, and backwards completely at will. Make sure to practice both sides!
Another way to view fore-aft distribution is to analyze what goes wrong when our distribution is incorrect. If your weight is too far forward, the ski will tend to twitchy and oversteer in turns with the tails washing out. An obvious sign would be falling forwards over your tips when you hit a bump. If your weight is too far backwards, the ski will tend to run off on its own and be difficult to direct or control.
As a final note, the penalties for being too far back are generally more severe than being too far forwards. One of the most common serious injuries when skiing is an ACL tear. The mechanism for this is most commonly a backwards twisting fall, often from sitting back in response to something unexpected on the slope. Staying forward in a good aggressive stance and skiing in control will help avoid this scenario.