This guide assumes everything you make is sold to a vendor. If you have an enchanteralt, you can send the green items you make to him for disenchanting, however this guide does not combine leveling enchanting and tailoring. This guide also doesn't tell you how to make money with tailoring.
This guide does not tell you how to get all the different recipes from quests, vendors or drops. What you do once you hit 375 is entirely up to you.
This guide is primarily aimed at those who want to switch profession at high level.
Side note: If you don't want to travel around then the only trainer you need is one of the Grand Master Tailors that can be found in the starting areas of Northrend (see Tailoring trainers for a list). They teach all levels of tailoring (from Apprentice to Grand Master), and sell every thread and dye required.
The recipes used require vendor bought components such as threads and dye. These cost gold. To learn recipes from a trainer also costs gold. The amount of gold needed is approximately 150 if you farm all of the cloth. If you want to buy everything you need from the auction house, expect to spend upwards to 250 to get tailoring to 300 and an additional 1200-1500 to get your skill up to 375. This does not include gold for recipes from vendors or the auction house.
You are going to need a lot of [Linen Cloth] and [Wool Cloth], huge amounts of [Silk Cloth], [Mageweave Cloth] and [Runecloth] and an insane amount of [Netherweave Cloth]. Below is a table that shows the amount of cloth needed. The minimum amount column shows the minimum amount needed if you would get a skill point every time you make something. However, at some point a recipe will be yellow or even green when you make something, so you won't always get a skill point. The estimated amount column deals with this, so try to get the estimated amount of cloth. If you are buying all the cloth you need from the auction house, get the minimum amount and only buy more when needed.
This guide uses trainer-available recipes, because they are the easiest to get. However, sometimes vendor-bought or dropped recipes are needed or simply cheaper to use, so there are some exceptions. Below is a table with those exceptions. Check the Auction House for the dropped recipes. The vendor for the [Imbued Netherweave Robe] and [Imbued Netherweave Tunic] recipes belongs to the Scryers faction. However, the recipes are not BoP, so ask a guild member or friend to buy them for you if you are Aldor. You can also buy the recipes if you haven't made a choice yet and are still neutral to both.
Use the bolts set aside earlier to make [Linen Bag] to get from 50 to 75.
There are two other recipes that can be used to get from 50 to 75: the [Heavy Linen Gloves] and the [Reinforced Linen Cape]. These recipes are cheaper in components than the [Linen Bag], but since the bag sells for more silver at a vendor (2 for the bag versus 29 for the gloves and 67 for the cape) and you made a ton of bolts to get to 50 anyway, making the bags and vendoring them will help you recoup a little bit more of the cost.
At 300, make [Bolt of Netherweave], which turns gray at 325. You are going to need at least 430 bolts to get to 360, so this should get you to 325 just fine.
At 325, you can make [Bolt of Imbued Netherweave], which becomes gray at 340. You are going to need at least 90 bolts to eventually get to 375, so make those. If you are not at 340, make some more bolts until you are. The bolts are needed in most high-level tailoring recipes, so they are not wasted. However, if you do not want to make more bolts than needed for this guide, make [Netherweave Boots] for those last points to 340 instead.
If you are a caster who is trying to make the 3-piece purple BoP set, make the first piece at 360, and the second piece at 374 (it's still orange, so it might save you multiple attempts), and you will save the cost of making at least 1 [Imbued Netherweave Robe], which are well over 25 worth of materials each.
If you are an enchanter or you have a friend or guild mate who is, disenchanting the uncommon and rare items that you manufacture can help recoup the cost of the materials, and can also be an aid in leveling the enchanting skill itself.
An alternative if you're just trying to get to 350, for [Primal Mooncloth]/etc, do the following. You should make a lot of imbued netherweave for skillups, as you will need 3 every 4 days to make your [Shadowcloth]/etc..., so even though it's expensive, you need it anyway. This also lets you wait a week or two and make cloth to get from 350 to 360, saving you some other funds.
Cataclysm is required to continue past 450. High level WLK recipes will provide skill level-ups past 450 with Cataclysm. Some Cataclysm recipes will increase skill more than one level. The following list of Cataclysm items can be crafted.