As for tips that aren't in the sidebar/wiki, my top tips would be:

Don't buy a kit. It might sound appealing to have someone else curate your initial investment, but all of the kits are currently designed for those who measure by volume, not by weight. Measuring by weight is fast, easy, and has essentially no clean-up or fiddly bits to deal with, which makes it a fantastic way to start out. Many kits also include non-standard flavourings often intended to be used individually instead of as components in a recipe, which limits their popularity and the ease with which you'll find recipes and advice online.

Don't order flavours that sound good, or the flavours in the first-order lists or popular flavour lists. Instead, collect a list of recipes that sound appealing to you and ideally have lots of reviews and discussion, organise them by similarity, and order the flavours that allow you to make a reasonable selection. One you have those flavours figured out it's fun to toss in individual flavours that sound appealing, but always have a plan: if you're making a strawberry lemonade, perhaps try that recipe with raspberry as well, but if you're not sure what you'll do yet with Kentucky Bourbon, it might be best to wait until inspiration strikes.

Flavours can be very, very dissimilar across companies, and it's best to think of them as specific, discrete ingredients that cannot be substituted with an expectation that the outcome will be the same. In addition to taste differences, there will likely also be a difference in strength. By all means, use what you have on hand or what appeals to you (experimentation is the name of the game!), but be aware that changing the ingredients will change the final product.

And finally, the number-one suspect if a recipe comes out too weak is over-flavouring. It might be counterintuitive at first, but there comes a point where adding more flavouring only dulls the taste. Watch out for older recipes: they might have been designed for use on less efficient devices like cartos, and may contain way too much flavouring to be effective on modern devices.

As for YouTubers, /u/enyawreklaw maintains the website DIY or DIE which has mixing information in a variety of media, including videos on his YouTube channel. He's an active member of the community and has released many delicious, well-rounded recipes with thoughtful construction notes; his Bronuts recipe in particular has been quite the sensation.

starting out, you'll not be able to resist ordering the 4oz bottle because of the price difference (just pay a few dollars more, i get more than triple the amount!!).... never fall for that, try to stop your self, when testing out a flavor for the first time order the smallest amount possible (even if it'll cost you the highest/ml)... flavors are so diverse and you will not know what they're like and if you like them before trying them out.
lets say you want apple flavor... the variety is from artificial apple-like flavors (think kool aid) to something closer to a real apple, with many in between. To get the closest feel to eating an apple you will need to mix a number of flavors.

I have many 4oz bottles that i haven't used for over a year from my first ever DIY vape mail. I may have spent the lowest/ml for these flavors, but the total amount was a waste because they ended up being donated/thrown away.

What I ended up doing was looking for recipes I wanted to make first. e-liquid-recipes.com is a great place. Sort by rating and find the ones you think you will enjoy.

Once you have your recipes, order just those flavorings. In the smallest quantity possible. Once they come in, you will be able to make your recipes and give them a taste test. After that, you will know what to order in higher quantities.

I hope it works out for ya!

I recommend getting some of these for mixing up a recipe: https://www.amazon.com/dp/B00SS1SNOS I use a straw cut at the end to form a fan then drill my mix to test right away. If it's good, let it sit for a week, if it's bad they're easy to dump and clean out for reuse. Previously I was using my 10 or 30ml dropper bottles for new creations but have wasted more of these than I care to think about.

For testing out a single flavor I use a 5ml bottle at 30/70 PG/VG, in something like these: https://www.amazon.com/dp/B00KRBM6IQ they're cheap and you can dedicate a bottle to test each of your flavors. I would start on the low end of the recommended percentages and then play with the numbers from there.

In addition to what others have said, get a composition book.

To start, mix a simple 2-3 ml of each flavor individually at 5 and 15% in the vg ratio you intend to vape. Create a page for each in your notebook, and vape a little every couple of days for about two weeks. Take notes on flavor profiles, mouthfeel, and development of flavor (ie started weak then became overpowering at 10 days).

This data set is invaluable as it helps you create recipes later and you know exactly how the flavors develop so you will a) waste less and b) know exactly what if any age you need to give the juice.

Then write down every test batch as well. Every ingredient mix. Detailed notes save you the heartbreak of an awesome recipe you can't remember exactly what amounts you mixed in it.

Write down everything you do and label every bottle. You might make some really good stuff and not know how to recreate it.

If you are ordering from vaporeyes you can you this discount code RTW8-2XDK-9G1D for 5% off its not much maybe an extra bottle of flavoring. Its valid for another 10 days.

Shake the holy twinkies out of your Nic base before you add it in. Seriously shake shake shake shake spin turn shake shake shake then when you think its good do it all over again.

I'm not sure this will appeal to others, but I'm very happy I did it. Make a (don't let the next word glaze your eyes over) excel spreadsheet with your flavors with three columns: manufacturer, flavor, and amount. Sort them by flavor. Then copy the full three columns and paste them to the right of the first three so you have two copies. Then select the whole second set and sort by manufacturer. Every time you unpack a new order, add them to the bottom of the left list, and sort; copy, paste over the right list and sort. Now you can browse by mfr or by flavor or by flavor within mfr. Now you won't order something you already have, and you know what size bottle you're looking for, and you can tell at a glance whether you have the flavors for a recipe that looks good (and often if you're missing one you can see what substitutes you may have to work with).
I know ELR can keep track of your flavors but entering them in is more hassle (imo) and this spreadsheet is (imo) more accessible.