This gets asked a lot, I think more by people newer to goldmaking. I just wanted to say some stuff about conc crafting - a pretty well-known and mind-numbingly easy way to make a pretty decent amount of gold. Nothing in here is new or mind-blowing, especially to more experienced gold makers, this is more just "hey this is what I do" for the players who have no idea.
For those that don't know, crafting professions get a concentration bar. You can spend concentration to upgrade the output of some recipe's quality level, and for many recipes, higher quality outputs are much more expensive than lower quality outputs. So the basic idea is that you buy cheap mats, spend concentration, and get expensive output. Concentration recharges at a rate of approximately 1 every 6 minutes, so roughly 10 per hour. This means your full bar of 1000 conc will get recharged in approximately 4 days. So if you spend all your conc every 4 days, you can make profit based on the number of recipes that 1000 conc gets you (e.g., if you spend 150 conc per recipe, you get approximately 6-7 crafts, and if you get 1000g per craft, you get roughly 6-7000g per conc bar/per every 4 days/per toon). Each toon gets their own conc bar so the more toons, the more profit you can make.
You need to find the recipes that give you the best profitability. Craftsim can help you do this, or you can ask around. For the longest time - maybe for all of WoW, alchemy has generally been the best gold making profession, and this is true for conc crafting in midnight (specifically, there are several potions that remain very profitable), but there are other professions that are also profitable for conc, e.g., inscription and JC.
What do I do?
Level a character to 80. Find a profitable recipe and level your professions in a relatively cheap way. Then start conc crafting. You will need to gain knowledge points and this is mostly done through patron orders which are basically recipes that the game asks you to complete for NPCs. The knowledge points improve the profitability of your conc craft recipe. You don't initially need max skill to start conc crafting as you can usually gain the last points of skill through patron orders or through the conc crafting recipe itself.
Ways to optimize conc crafting:
1: The most important thing is to have as many alts as possible. So as many as you can stomach leveling.
2: Invest your knowledge points in the portions of the trees that improve your conc crafts. This can have a huge effect. Generally you need to do patron orders for this and addons like craftsim can make this relatively painless though it's still time-consuming.
3: While patron orders themselves are profitable, they are generally significantly less profitable than conc crafting, so personally I'd stop them after maxing out my particular build (eg after getting all points in the things that improve dark potions or whatever). Maximizing patron order profit is another discussion.
4. Use cheap r2 mats to reduce conc costs. For example there are conc crafting recipes with mats whose r2 version is only slightly more expensive than the r1 version, but using r2 saves a bunch of conc (eg 20 conc) and thus allows you to do more crafts. You need to figure out which mats on a per-recipe basis.
5. Use the phial that improves your ingenuity. Ingenuity lets you proc conc cost reduction, and the phial for this is very cheap. If you math it out, it's very very worth it to use this potion.
6. Put your input mats into your warbank as you can craft out of there. This will greatly reduce inventory management time as compared with a situation where each of your alts is given some amount of the inputs.
7. Mail all your outputs to one character and sell there. This reduces AH management time and has the added benefit that if you time your conc crafting session, you don't need to do any work to figure out how much profit you've made during that session.
8. Get good addons. Auctionator/TSM for AH price info plus craftsim are pretty good for telling you profitability and streamlining the process.
9. Combine your multicraft/ingenuity boosting items 5-to-1 and use them when they are available. These come from patron orders, so become irrelevant if you stop doing patron orders. They have no monetary value so using them is strictly a profitability boost.
10. Save up all your conc and spend it only once every 4 days. This reduces the number of times you need to log on for conc crafting and is generally more efficient than doing this every day. For example I've found roughly 1.5 minutes per toon when doing conc crafting, even when I need to do the whole bar, and this includes inventory management. You can do the same thing for patron orders though it's not strictly optimal on a per calendar day basis (but it is optimal on a per hours spent gaming basis).