I don't honestly see it being all too important a step with this particular model, but I'm going to cover this step because I use it with all my other texture painting.
Whenever I want to add in shades or highlights to a texture I NEVER do a direct burn/dodge onto the texture itself. If you do that, you can't smudge, erase, or adjust it without messing up your texture image underneath. So instead I use adjustment layers.
Click on whatever layer you have directly underneath your UV reference layer - it doesn't matter what it is, all that matters is that it's 2nd from the top.
Now Click on the adjustment layer button at the bottom of the layers window. A menu will pop-up from that. Choose Hue/Saturation... from the list and it'll create a new layer and bring up a Hue/Sat window. Pull lightness way down so everything is dark, and adjust the hue slider a little to one way or the other - but just a small amount.
Click OK. The way an adjustment layer works is with Black and White. If you look at the layer itself, the little adjustment icon is all white right now. We need it to be black so Go to Image > Adjust > Invert. Now your whole texture looks normal. If you choose the paintbrush tool, you can only paint in greys. White to Black and anything between. If you pick a grey around the middle and paint on the layer, it'll put in about 50% of the hue and lightness changes you set before. Paint with white and it does the full 100% of the adjustments. Every layer underneath the adjustment layer is affected by it, but you can paint, smudge, and mess with this one as much as you want and it will never physically change the laters below it