The most important step in being able to buy a mount and make other large purchases should be self-evident: saving. Economize as often as you can, and don't buy anything unless you absolutely have to. You can burn through hundreds of gold even before level 20 by visiting the auction house for new equipment at every opportunity. If you do so, over the long haul you will be left with very little to show for it. Before level 20, keep your eyes on the prize: getting that mount. The mount helps you move faster. Faster movement means faster killing, faster questing, faster quest turn-ins, and faster leveling. It is the most important tool to fast leveling you can get at level 20, and infinitely more important than getting your hands on that Left-Handed Vorpal Cleaver of the Zipswitch that you could have purchased at level 23. Stay focused.
Flipping is the least time consuming way to make gold in WoW. It simply involves trading on the auction house. You buy low and sell high. There are a lot of different markets you can try your hand in flipping both Legion and old world. Flipping works best when there are natural variations in either demand or supply for an item which will cause the price to vary over time.
To use it as a bank, figure out how much you want to have on-hand on your character based on how much you normally spend on repairs, food, ammo, etc. and send the rest to the bank alt. The principle here is "Out of sight, out of mind.". Money "you don't have" cannot be spent, requiring you to log out of your character, and then to log into the alt.
jame, addon version or your guide is F$%&ING AWESOME, THANK YOU. there is a part in the first section of the guide where im instructed to take shindrell's not to kayneth stillwind, but there is no instruction to pick up that quest on the way through astranaar, if i find anything else like this ill let you know, love your guide, thanks for taking the time to put it together!!
Professions: This may seem strange for a leveling guide, but the first opportunity you get, pick up both the Mining and Herbalism gathering professions. Not only will this allow you to track many various quest objectives on your map, but you also get a fair amount of experience whenever you gather. I'd put it at around 10% of a quest turn-in per gather. Since most of your wasted time in WoW is travel time, stopping to gather ore or herbs can keep your exp/h rolling as long as you don't go too far out of your way. You should only gather if it's convenient, and on the way to an actual quest objective. Don't make gathering your priority.
If you want to spend as little time on your alt as possible - just do Legion Invasions. You can use sites like (https://wow.gameinfo.io/invasions) to track when the next invasion will be. Log in, fly to the closest world quest, and once there - right-click on the quest's name in your quest log and "Find Group". Sign up for some group, or if there's none - just start your own immediately, and people will come to you. It is 10 times faster to do invasion quests in a group. Once that particular quest is complete - leave the group, and fly to the next one, where you should find another group. Don't expect the group from one world quest to follow you to the next. Then you simply log out until next invasion. You can do 1-2 invasions a day, and each invasion will give you about a level or 1.5 of experience, if you have full heirloom gear. Each invasion will take about 20 minutes.
Anything that can be answered by reading the recent discussions on the subreddit, has a yes/no response or can be looked up on sites in the community resources should not be posted in a thread of its own. Questions such as 'What is this worth?', 'Did I make a mistake?', 'What do I do with XYZ?' or 'Should I buy/sell this?' will be directed to the Daily Questions sticky.