I'm pretty new to DIY mixing myself but I can tell you what Ive learned.
Attys -
Most mixers will tell you to use an atty and drip for testing flavors instead of a carto. This makes sense since you don't have to fill a cartomizer with a mix that might not taste good. One tip I learned while using an atty is that you can drip and vape straight PG in between flavors to help flush out an old flavor if your switching between two of your mixes.
Also dewicked attys retain less liquid and are better for testing different mixes.
Amount of flavor -
Im still struggling with this myself, but what you'll generally hear is between 5 and 25 % total flavor in a mix. In some cases perhaps higher or lower. In the last month the highest Ive heard someone mixing at was 25 %, but Im sure some mixers go higher. Flavor Art advertises that their flavors are more concentrated, and to start at 1% for their mixes.
The only thing I have from FA is the Vape Wizard, so Im not sure yet about what flavor percentages work best with those but I can tell you that my experience thus far with The Flavor Appretice is that my first two flavors were weak. The chocolate I mixed at 10%, then at 14% and it was weak at both percentages. The Cappachino Ive mixed at 18%, its much better but still a tad bit weak. TFA is advertised at 10-20% flavor BTW, for me I'm thinking I'll be mixing strong on most batches if this pattern continues.
Each flavor will probably be different though so finding a good percentage for each will take some trial and error.
Steeping -
Many mixers say that steeping your mix by letting it sit for a few days changes the flavor a great deal, others say thats bull.
In my experience so far Ive noticed that steeping darkens the color of the liquid, but in my first mix I didn't notice any change in flavor after a week of steeping. This opinion could change, since Ive only done a week long steep once, and I plan to try it again.
MTS Vape Wizard -
Ive only tried this once so I really don't have an opinion on it, I did use it on my first mix at .5% but it didn't save my chocolate which was simply lacking in flavor. I heard one person say they don't really use this on creamy flavors like chocolate so I may have tried that for nothing.
Sweatener -
This is something Im in the process of getting right now. I need something that works great as a sweetener. After a couple of forums and several questions Ive heard about ethyl maltol being great for sweetening. You can buy this already diluted in PG from The Flavor Apprentice, look for (Cotton Candy).....
flavorapprentice.com
The Flavor Apprentice Professional Flavors - One Stop DIY Shop Store
It's used as a mild sweetener or to smooth out a harsh tasting liquids. I haven't tried it yet but many people have recommended it to me so I'll probably get it.
Hope this helps a little, I'm still learning myself.