Alchemy is a great profession for gold-making as the main outputs are consumed on use, this means that there will always be demand for new potions and flasks. The old world recipes are also very profitable. Sadly I don’t have Legion alchemy so I can’t personally help you. The best guide I was able to find on the internet is this video guide by Ninja Kuma.
The majority of you (myself included) don't find farming fun. You don't have 20+ hours to burn each week on an activity that you find dull and mindless. Because of these things alone, you may try but you'll never be able to commit to doing this week in, week out, and this is why you'll fail. Repost after repost, without seeing success, you'll simply call it, and be just another guy who saw a transmog youtuber, tried to get into the transmog business, and quit after a day/week/month.

I tell you my experience. I'm leveling a priest that it's 60 now and since I have also done some instances (and I have full heirloom set with 20% experience gain bonuses) and been very ahead of the guide, I skipped some zones since they were grey quest for me, well it was a real mess. Missing quests because I didn't to the previous to enable the following. Going in a zone then realizing that I couldn't do that quest because I didn't do the starting quest chain, very unconmfortable. In short what I suggest you it's not to skip areas, or make really sure that you do all the quests required (at least) and check carefully the previous zone you'll going to skip to make sure that you won't have problems with struggling finding where to start the quest chain to sync with the guide section.
If these tips are not up your alley, feel free to play the auction house as early as you have enough to buy one cheap stack of peacebloom. These low level herbs are fantastic sources of early income with little risk since everyone needs to level professions. Alchemists especially are popular every expansion because of the need for those high level trinkets that soar above even heroic drops. As a result, that peacebloom you bought for 10 silver now can easily sell during peak hours for well over a gold! This is a minimum of a 1000% profit! While 1 gold may not seem much, when you are selling 10 stacks at a time, those few hundred gold can go very fast and get you into the greater profit items. However, knowing prices on peacebloom and other lower level mats can prove difficult. This is where Auctionator finally comes in! Run a scan overnight on all your auction characters. You can now see whether or not a posted item is worth buying because of percentages presented next to the list. The major lesson to take out of this is to scan daily and see which items fluctuate greatly, as those are your rewarding items! Items that jump from 10s to 3g depending on time of day or uninformed buyers are a goldmine waiting to be taken advantage of. You can buy low and sell high, succeeding in the World of Warcraft stock market!

As always, I will monitor and update this guide as new information comes in. If I've gotten anything glaringly and completely wrong, just tell me with as much data as possible. The idea is to find the fastest possible leveling method, NOT to argue about how good or bad individuals are. If you have hard data, post it! If you have an idea, let the thread know so we can test it!
KUI Nameplates offer the highest degree of customization and accuracy. For example, you can easily configure the nameplates to enlarge and change color when a target reaches a certain percentage of health, which can be very helpful for classes with executes. They always correctly display your debuffs, and their click box is very accurate. Some setup is required, although it is very simple and straightforward.
Make sure you have the two Flight-Path toys purchased, as this can save you time. It won't have EVERY flight path, but will have major locations for faster travel if you need to switch zones entirely(which you will). If you buy the toys on one faction, it unlocks for the other faction as well. So don't spend double the gold! These toys can be found at the heirloom vendor(Undercity for Horde, Ironforge for Alliance)
Leveling used to be a treat in WoW. New, unique abilities drove you forward and made getting past the horrible grind worth it to some extent. Now it just feels like everything is set to grind. More than it used to be. Once it was tolerable, rewarding even, but now you grind to unlock an allied race through reputations barely connected to most of these races, only to have to level up that character to get their unique armor set (compelling you to not just boost your character and instead suffer through the slog of ANOTHER group of alts), then you get back to current content and grind some more with a random chance to drop the item you need to get your ilvl up to finally enjoy the single warfront available (as I write this).
The most simple thing is TradeSkillMaster. This AddOn shows you quite an amount of information in the items tooltip. In my opinion, DBRegionMarketAvg the best source. It tells me the price, an item was posted on all EU realms in the last 14 days. Why not DBMarket? DBMarket only shows you the items value from your server in the last 14 days. This value often gets manipulated. Some people start to post an item with a 500g value for 500.000 Gold on the auction house for several weaks. As a consequence, DBMarket is rising towards 500.000 Gold.  If you use DBMarket with 100% and post your item for 500.000 Gold, nobody will buy it.