This was originally written for my guildies and is a rough guide to 425.
So Leatherworking in Northrend...ugh. I've not found a good guide to powerleveling yet, so I decided to try a hand at one.
ETA: The important thing that I got wrong from wowhead is that heavy borean leather is not actually trainable until 390. What I did to level was the following:
375-380: 6x Borean armor kit (turns green at 380)
380-385: 5x Iceborne Gloves
385-390: 5x Nerubian Legguards (only did this to have some variety)
From 390-405 I made heavy borean and fur linings.
From 405-410 make Arctic Wristguards, which are 12 normal leather each and go green at 410.
From 410-415 make Jormungar Leg Armor, which are 5 heavy borean and a jormungar scale each. And hopefully they'll sell decently. Also around this time get some arctic fur and make your fur linings, since that should ding you. If you have legs that are worth a damn you can get a couple of jormungar scales or nerubian chitin and make your cheap leg reinforcements too.
From 415-422 make pack of endless pockets, a 32-slot inscription bag. This takes 8 heavy borean. This is the only one that's trainable directly; the other nice bags (mining, leatherworking, and the quivers) require rep. If you have the rep, I'd recommend doing the leatherworking bag instead, as it requires kal'uak revered which you might have that early.
From 422 on it gets really sketchy.
From 422-430 (you can stop at 425 and do the next one if you want better chances, but it may be more expensive) do eviscerator's bindings or stormhide wristguards, depending on whether eternal air or water is cheaper/easier for you. They take 8 heavy borean + an eternal air or water. At 425 I'd also recommend pimping out yourself and looking for work so that you can get skillups on frosthide leg armor and icescale leg armor with their mats. These take two arctic furs and either two nerubian chitin or two icy dragonscales. If you're lucky or persistent, that can take you from 425-440 by itself. If you're unlucky or impatient, you'll need to make things like the eviscerator treads and eviscerator waistguard to 435. These should sell well at least, since there will be lots of people who want a starter pvp set. They take 10 heavy borean + an eternal air/water each.
At 435-440, about your only reasonable option is to make more leg armor patches. You can make more eviscerator gear, but it goes green at 435 and gray at 440, so you're looking at making a LOT of those.
And then you're done doing it for practical reasons. 440 is the max leatherworking you need for any recipe. Leg armor goes green at 440 and gray at 445, so past 445 the only things you can make are items that require frozen orbs.
Anyway, that's what I'm looking at doing. If you can get to 440 using just their mats, you're basically looking at the following mat requirements to get to 425:
838 borean leather (of which, 80 will be used to make actual items)
8 jormungar scales
3 eternal waters/eternal airs
+ an arctic fur for your wrists and scales/chitin for your legs, if you so choose.
If you're going from 425-435 using your own mats, you will also need:
130 heavy borean leather (or 780 borean leather) + 13 eternal airs/water as desired.
If you're going from 435-440 using your own mats, you will also need:
14 arctic furs + 14 nerubian chitin or icy dragonscale as desired.