Since everyone is offering their experience with DIY, I will too. YMMV.
1. I don't steep or age any of my DIY liquids since I start vaping them almost immediately and only make in batches that will last me two to four weeks (100 - 200ml). with only a couple of exceptions.
2. Those exceptions are flavors dissolved in alcohol (needs to be aired out a bit unless you like the increased throat hit), tobacco (needs to be aged, but I no longer vape tobacco flavors) and some creams (will mellow over the course of a couple of days after mixing).
3. Unless you have very expensive, lab-grade equipment, there's a pretty significant margin of error when mixing (by volume or weight) no matter how careful you are. I mix by volume with syringes.
4. When mixing a new flavor recipe from scratch it almost never comes out exactly as you think it will but it occasionally will come out surprisingly good... then you adjust.
5. If you've read through the link I posted, you'll notice a 100 drop test for approximating flavor profiles in any new recipe. it works well and I'd recommend it.
6. For sweetener, I use TFA's cotton candy flavor (ethyl maltol). It generally vapes sweet but doesn't taste sweet if that makes any sense. using it doesn't overly gunk up your coils (although ALL liquids I've vaped create some kind of gunk after awhile). for sweet taste I rely on vg.
7. Storage of your component ingredients is key, especially with nicotine. Keep in a cool, dark, non-reactive container (glass) with as little air as possible. I find my flavors last (in PG) last for a long time even with air in the bottles.
8. I've found that I can store components rather indefinitely and they only start to change when I mix them into a finished liquid in a ready to use container. I usually keep my all day vape liquids mixed in a flavor-only glass jar 120ml. when I need more liquid, I just mix one completed flavor with nic, pg, vg, and go.
9. Pre-mixed/finished flavors are usually the only single-flavor mixes I make. Otherwise, most recipes have a primary flavor, one or more secondary flavors, and a cream. I was surprised to learn about the cream. it's good to have a number of cream options on hand. vanilla is especially useful, too.
10. DIY is very rewarding if you have the knack for experimentation/testing. Recipes change with different ratios of pg/vg and devices but after awhile you learn what adjustments to make just for your taste, desired vg/pg ratio, and even the power/temp you vape at.
11. when I'm experimenting, I keep an atty that's easy to clean/wick for testing. I generally use a Heron for that (without a topper and no fluff) just to be able to test a few drops at a time.