I promised to create a tutorial on how I painted my Imperial Fist from a few months back. Hopefully this'll give you all the information you need for beating the dreaded beast that is yellow on black undercoat!
Note: This is a living document that I'll expand upon and increase the quality as time goes by. The basic tutorial will be finished tomorrow evening (Friday 29th January '16) but I'll be adding to it as time goes by so make sure you follow or check back every so often to get the latest updates.
STEP 0.0
Get yourself a model that you'd like to paint (ideally a Space Marine with his boltgun unattached). Here's one I made earlier:

STEP 0.5
Find a clean, comfortable and well-lit area where you can paint in peace. Here's a bad example. Don't do paint in such messy conditions, folks.

As I said, baaaaaad. And yes, I use my table as a palette. Again, not the smartest of ideas.
STEP 0.75
Undercoat your model black.

I personally use Citadel Chaos Black spray paint. Make sure you get an even coverage over the whole model.
STEP 1.0
Take your pot of Citadel Base Averland Sunset. Give it a good shake. Now, take a bit of paint onto your brush, quickly dip it into the water of your water pot just to dilute the paint a bit and then slap it onto the model. Repeat until the armour is covered. You should end up with something like this:

Yes, it looks terrible. The key here is to go with multiple thin layers of paint so as to not obscure any detail and to have a smooth finish.
Now, add another layer of Averland Sunset all over.

Already looking better, yes? Now, another layer. Keep the paint thin but not too thin. We don't want it runny.
+ Top tip: Just before you add another layer, close your paint bottle and give it a good shake. +

See? It's looking pretty good already. There are still some dark patches though, so we need another layer.

That looks about right. The black still shows through in some areas, but I can assure you that those areas will be covered up later on. Please ignore the fact that I almost forgot to take a picture of this stage and had already started putting a wash on the model's right leg...

STEP 2.0
Now, take you pot of Citadel Shade Seraphim Sepia. Shake it well. Dip your brush into it, then into the water pot (quickly) and then paint into the recesses.

Be as clean or as messy as you wish. You'll have to clean up the later steps anyway, so it's up to you. I went for rather messy. Just make sure it doesn't pool anywhere it's not supposed to. If it does, rince your brush, dry it and quickly remove some of the wash.
STEP 3.0
For this step, you need a pot of Citadel Base Abaddon Black (HERESY!


As always, multiple thin layers are better than one thick one.
+ END OF PART ONE +