This is something that I've done for a while, but you can do with little to no experience. If you know how to use tweezers, you can re-pin a lock. Once you get used to it, you can re-pin a lock in 5 minutes with no hiccups.
I sometimes have issues with the process, and have never been able to find a good tutorial online; so, here is a rather in-depth step-by-step guide for those of us who need more visuals.
The tools you'll need are:
Kwikset Re-Pinning Set
Doorknob with lock.
Step 1: Accessing the Tumbler to Be Removed
Most re-pinning sets come with something called a "Removal Tool." It's a long, Y-shaped piece of steel that is one of the most useful parts of the kit.
Insert the thin end of it into the spindle portion of the knob. It's a vaguely cylindrical piece of metal with a half-cylinder surrounding it. Twist the end so that the tab at the base of the spindle is perpendicular to the half-cylinder (Pic 2).
Then, take a small screwdriver (or a larger one, if you can fit it in) and find a recessed little tab directly behind the half-cylinder (Pic 3). If you push it in with the body of the screwdriver, it will unlock the spindle and you can pull it out (Pic 4).
Once more, take the removal tool and insert it into the whole where the spindle once was. It might take a little wiggling, but if you insert it perpendicular to the half-cylinder, it should get resistance, and if you push a little, it should feel springy (Pic 6).
If you flip the entire thing you're holding upside down and push it onto a flat surface (Pic 7), you can then push down and the tumbler/cylinder portion should come out (Pic 8).
Step 2: Accessing and Removing the Old Pins
Pic 3 exists as a step, just because it's easier to take out all the pin springs before everything else so that when you turn the tumbler upside down, they don't go flying everywhere. Then you remove all the top and bottom pins by turning it upside down and shaking/tapping it.
Quick lesson in locksmithing. The pin springs, top pins and bottom pins all make up how the lock works. The top pins and bottom pins line up along the "shear line" when a key is inserted, allowing the lock cylinder to spin freely. When the key is aligned and removed, the pin springs push the top and bottom pins down, locking the cylinder in place. Locksmithing 101. Quick and easy.
To completely disassemble the cylinder from the tumbler, have all the pins removed and push the plug clip up so it is no longer locked using the funky-looking cap removal tool. You can see it in Pics 4 and 5. It is an oddly-shaped piece of sheet steel with a little tab at a 90º angle that hooks into the plug clip so you can pull it out without damaging fingernails.
Once that is done, you have all the parts of the disassembled lock laying before you.
Step 3: Figuring Out the New Key's Pins
But first! You need to figure out what the pins are for your new key.
Using the pin gauge shown in Pic 1, you find the depressions in the key between the little tooth-shaped parts that are rather sharp. These depressions match up with the pins you're about to put in.
Remember the order and write down the order of the pins. They start from the tip of the key going in to its shoulder (the part of the key that stops it from going into the lock all the way).
Your pinning set should have all of the pins labeled to match up with the pin gauge you have. Pic 2 has the variety of stuff in the full-sized pinning set, but along the bottom are the different sized pins.
It helps if you use the tweezers to pull the pins out of the bag. They are oily and it helps to not get that oil on your fingers while working. It also helps not to get anything on your fingers on the pins. (Pic 3).
Step 4: Adding the New Pins
First, you need to put the new pins in, in the correct order. An easy way to do this is to put the key in the cylinder with the depressions in the key visible as shown in Pic 1. This gives you a visual so you can get a general idea of what sized pins go where. You already have the correct numbers written down (hopefully) so everything should match up.
Once all the bottom pins are in, you should be able to insert the key up to the shoulder, and the pins should lay flat along the shear line (Pic 2).
After that, you insert the cylinder back in the tumbler and put the plug clip back on. This will make it certain that everything lines up properly (Pic 3).
Before reinserting all the bottom pins, you'll want to turn the cylinder so that the key is perpendicular to the pin channels...or whatever the technical term is. This will allow you to line everything up without some falling deeper into holes and causing small headaches (Pic 4).
Once the mechanical portions are reassembled, you can start adding back in the top pins (Pic 5). Note: the top pins are almost completely cylindrical, versus having a slight bevel around the edges as the bottom pins do.
Then the springs go back in. These you can touch with your fingers. The tweezers aren't kind to thin spring steel (Pic 6).
Once all the tiny bits are back in, you can remove the key and you'll see rough indents that are inverse to the key's teeth. As shown in Pic 7.
Finally, put the little steel cap back across the pin channels. This will keep everything in and keep spring tension on the pins. Beware, try to keep the pin springs in line when you put the cap back on, because otherwise you might bend one and cause it to act and look like a broken Slinky (Pic 8).
And now the actual lock mechanism is reassembled! Go you!
Step 5: Reassembling the Whole Thing
Depending on the type of door knob, there should be two semi-distinct channels running in the inside of the knob portion. You can see them in Pic 1. They're hard to see, but the channels are the darker portions that didn't get reflected in the flash.
Lining the two copper-colored tabs on the back of the tumbler so they fall into the channels, you can push the tumbler in until you can feel/hear a click or two. That means it's back in. (Pic 2).
Once you're done, you can reinsert the spindle that was removed near the beginning of the process. You shouldn't have to do anything besides just push it back in the hole, and it will self lock.
Step 6: Put the Lock Back on the Door and Enjoy!
Congratulations on learning how to re-pin a lock without having the key!