You can fill your own cartomizer. You want to take off your cartomizer from the battery before filling it. You also want to make sure there isn't any juice leaking around the cartomizer before screwing it on to the battery.
You should also consider buying your juice from a separate vendor. Here are a few vendors you might like: Vapor4Life.com, HaloCigs.com, JohnsonCreekSmokeJuice.com, Avejuice.com (Alien Visions), GoodProphets.com, TastyVapor.us, GourmetVapor.com, BackwoodsBrew.net, VermillionRiverEjuice.com.
When you're filling your cartomizer, I just drip in from the sides. It takes a while for the juice to soak into the filler, so I usually find myself rotating the cartomizer while I fill it so I can get more drops in before I have to wait to let it soak in. You should be able to fit 25 drops in a cartomizer easy. You really want to make sure the cartomizer is sufficiently wet without overfilling it. To force the juice down into the cartomizer quicker, you can either try a taryn spin (you spin the cartomizer in a lanyard and centrifugal force pushes the juice down into the cartomizer) or you can shake your cartomizer like a thermometer. When the cartomizer is full, the top of the filler looks kind of slushy and it won't absorb any more juice.
It's always good to keep some paper towels handy when you're filling. Make sure you wipe off any leaking juice before you put the cartomizer back on the battery. If you get some juice down the center tube, make sure you blow out the juice into a paper towel. If you overfill the cartomizer, you can try shaking the cartomizer into the paper towel.
It's also a good idea to periodically top off your cartomizer with juice rather than
vaping it dry.
If you're swapping a bunch of different juice flavors, I'd recommend you use a different cartomizer for each flavor of juice. If you find yourself trying lots of different flavors often, you might be interested in getting an atomizer and 510 drip tip for sampling juices a few drops at a time. If you go with the KR808D-1 kit, the 901 atomizer is compatible. A 510 drip tip will fit on both the 510 and 901 atomizers.