After several burnt cartos and learning the hard way this is what I found works best for me.
I put a round toothpick in the center airhole on top of the carto barely applying any pressure. This helps to prevent my liquid from dripping down the bottom of the airhole and down the connector.
For the Boge f16 and f17 which both use an "XL" size or 45mm long carto I put a minimum of 20-25 drops of liquid in it. You may have to put 10 drops at a time so the juice does not overflow out of the top of the carto. For the regular size (35 mm) cartos I do 15-20 drops. This process can take up to 30 minutes so be patient and try to plan ahead. The reason it is important to count the drops is so that you know that you've properly saturated your filler material. This is much better than using a visual method and saying "ok the filler looks wet enough" because you can get that look without saturating the entire filler material. Counting the drops will ensure that all of it is wet.
Once you've put your required number of drops in the carto and let it soak for a little while put it on a device at low power and take a couple of really quick hits to make sure it's filled properly. This will save you the heartache of putting it into a tank and filling it with juice before finding out you did not prepare your carto properly or you have a defective carto. Once you've verified the carto is good to go install it in your tank and fill with juice. Take a few primar pulls and enjoy your vape
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Again this is just what works for ME. If it works well for you too then I am happy I could help. I am sure though that there are many different methods preferred by other folks.
Edit:
I forgot to mention that if you've burnt your carto then toss it. If you did as I stated and just did a really quick hit to test it then it is still salvageable most of the time. If you've already taken a nice long burnt hit on one time to toss it and not waste your time.
