Hey CuppaJoe and Welcome!!! Thinner PG juice will soak into the poly quicker, so I add 4-5 drops all around the edge while tilting the carto and rotate while I fill it. Then I sit it down for a minute. I have a 100% VG juice that is extremely thick. I can only add 1-2 drops at a time to let it soak into the poly and I have to let it sit about 3 minutes before I add more. Also if your in a hurry you can "shake down" the juice like you would a thermometer.
In a new carto, I usually get 25 drops into it. If you fill slowly avoiding the center tube letting it soak in good you should not have any leaking. You can tell it is finally "full" when the poly looks mushy glazed on the top. After getting it completely full, I still allow it to sit about 10 minutes standing up. I then check the battery end to make sure I didn't overfill and have leaks. If there is juice on the battery end I clean it with a Q-tip soaked in isopropyl alchohol.
Like Saundhaus was saying I too love the whole ritual of being patient and taking my time filling cartos, it is soothing and relaxing for me. An empty carto will hold 1 ml of juice. You can use a syringe to fill quicker by just putting the needle down the side of the polyfill or you can use the "condom method" fill a condom that doesn't have the center post with just over half of juice then slowly press the carto into the condom with the battery end NOT the poly end and let it just sit there and soak it all up. Just make sure you clean the battery end of the carto after using this method.
Like jwbnyc said that raspy sound is the carto being over full just vape through it and it will normal out for you.
I don't really wash or clean my cartos. I dedicate it to a flavor and then just refill as needed. When a carto comes near the end of it's life it will start to taste airy, less vapor and less satisfying. When it is toast you will have a gawd awful severly burnt taste that you will never want to repeat. My cartos usually last about 2 months or 5-6 complete refills. They will last longer keeping them topped off instead of letting it get too low before refilling.
Also juices like Voodoo or darker juices will gunk it up quicker so those cartos won't last as long as the clear thinner juices. And if your getting "dry hits" your juice is either too thick and isn't wicking well, I have a 100% VG that is like this, when this happens you can shake the juice down or slow down on your vaping. I think we all tend to just keep puffing away like it is an analog. Like DaveOno reminds me, it's okay to put it down between toots.