Measure the width at the bottom of the shower opening. Transfer that measurement to the bottom shower door track and mark with your marking pen. If the track fits to size, move forward with installation. If it's too long, you'll need to cut it to the appropriate length. Using a hacksaw, carefully cut the rail at the mark you made. Be sure the rail is held firmly as you cut to avoid damage to the rail or the saw. Use a file to remove the burrs from all metal pieces you cut. Before you go attaching the tracks on forever, you need to place them temporarily and mark where they need to go. Place the bottom track along the base of the shower opening with the higher lip of the track facing outward. Make sure the track sits flat along the surface. There should be about 18 inch (0.3cm) of play at each end. Temporarily tack the bottom track into place with masking tape, then mark the position with your marking pen along the inner and outer edges. Do not remove the bottom track yet. The wall tracks should be pre-cut to size from the factory. Slide the wall tracks into position with the bottom track. Make sure the wall tracks fit the bottom track exactly. Use your level to check for plumb. Holding each track firmly, use your marking pen to mark the mounting hole locations on each wall then set the wall tracks aside. With a nail or a center punch, tap a small dimple into the marks you made for the holes in the wall tracks to use as a pilot hole. This prevents your drill bit from "skating" and damaging the surface. Drill your mounting holes using the proper bit for the surface of your shower. If you are drilling into tile, put a small piece of masking tape over each spot to be drilled. This will help prevent chipping. Also, drill deep enough so the plastic screw anchors will fit properly. Anchors won't be needed on fiberglass. Run a bead of caulk, about the thickness of a bead of toothpaste, along the bottom mounting surface. Center the bead between the two lines you marked when you measured and run it the full length of the gap as well. Then, firmly set the bottom track into place over the bead of caulk. Make sure the underside of the track makes contact with the caulking. If not, run a separate bead along the center of the underside of the track. Hold the track in place for a minute or two, taping it down to hold it in place if necessary. It should be try in five minutes at the most, then you can move forward knowing that it's secure. Line them up with the mounting holes and make sure they fit properly over the ends of the bottom track. If you've measured correctly and marked correctly, they should snap right into place. If your kit included them, place the rubber bumpers that come with most door kits over the screws and secure the tracks to the wall by using your screwdriver to turn the screws into place. Do not completely tighten the screws at this point, hand tight is sufficient. Swinging doors will need to be installed differently, according to the kit that you've purchased, so it's important to read the instructions that come with it and follow the directions accordingly. The door needs to be installed so it pivots outward, but for some kits that'll be on the left side and for some that'll be on the right side, and the mechanism works differently, depending. With some, the door will just snap into place, while screws will be used for other kits. In most swinging door kits, a strip of rubber will be slid into the wall track opposite the pivot point, held with screws in some cases. If you had to cut the threshold track, you'll likely also have to cut the top track, since it'll be roughly the same length. Make sure the track fits snugly, connecting between the two wall tracks, and is properly aligned between them. It should just snap right on top. Many door kits will have corner brackets that are attached with screws to hold the top rail securely. Defer to the instructions of your specific kit, if necessary. Finally, run a bead of bathtub caulk along all the points at which the tracks come in contact with the walls. Do this to both inside and outside surfaces to create a neat, watertight seal. Let the caulk dry thoroughly and wait at least 24 hours before running water in the shower to test your work. It should be dry in a few minutes, but it's a good idea to wait and let things settle before you try it out.
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One-sentence summary -- Cut the tracks to the appropriate size, if necessary. Measure and mark where the track will go. Drill the holes you marked. Install the bottom threshold track. Mount the wall tracks. Install the door swinging door. Measure and cut the top track. Seal any gaps using caulk.

A Baker's cyst occurs when fluid builds up in the back of the knee. This happens when something is wrong or injured inside the joint. However, this can also occur spontaneously without any trauma. Some people just get them and there is no known explanation. This cyst points to a larger underlying problem, like a torn meniscus, that needs to be found soon because the swelling can cause damage to the knee. Bursitis is inflammation or an injury to the prepatellar bursa. Bursa, a round, fluid-filled structure, help the kneecap move smoothly across other tissues without friction. If the bursa gets inflamed, it can cause surrounding tendons to become inflamed and pinched, resulting in pain. Bursitis can cause stiffness or pain in the knee, as well as tenderness of the knee when touched and pain with movement. Pain is usually worsened by bending the knee and improved with extending the knee. The knee may also be swollen and red. Bursitis can be caused by repetitive motions, like bending or stooping. You can also get it by putting pressure on the joint by kneeling for too long on a hard surface. Patellar tendinitis occurs when you do the same motion on your knee, like running or cycling. Patellar tendinitis is when the tendon between your kneecap and shin becomes inflamed. Pain is the main symptom of patellar tendinitis. The pain is located beneath your kneecap, near the place it attaches to your shin. The pain may surround a workout, either as you begin or after you finish one. Eventually, the pain will make it difficult to stand or take stairs. Arthritis of the knee occurs when the knee joint swells. Arthritis generally occurs with age or due to a knee injury. It can make everyday tasks difficult, like standing and sitting, walking, or taking stairs. Pain, swelling, and stiffness are common symptoms of arthritis of the knee. Osteoarthritis (OA) occurs due to age. It can start around age 50, but may affect younger people. OA occurs due to the wear and aging of the cartilage of the knee, which causes less protection for the bones as they rub against one another. Pain may also become worse as you go through your day. Rheumatoid arthritis (RA) is a chronic autoimmune disease that will not only affect the knee, but joints over the entire body. RA usually starts affecting people between 50 to 75 years old, it is more common in women, and pain is often worse in the morning and improves with moving throughout the day. Post traumatic arthritis occurs after a knee injury. Sometimes this happens years after the injury. Broken bones, ligament injuries, and meniscal damage can cause this type of arthritis.
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One-sentence summary --
Determine if you have a Baker's cyst. Decide if you have bursitis. Monitor for tendinitis. Check to see if you have arthritis of the knee.