For this tutorial I will be using BlubBlubWhat's Nerki
You could use any skin you would like for this
http://www.planetminecraft.com/skin/nerki---blub/

Get the blocks in-game closest to your Minecraft skin's colors
You don't need to shade as much as the skin so just the base colors would work
For the skin I am using I will be using these colors

We first want to start with feet
So whatever color the bottom layer is place it
It would be 8 blocks long
The entire of the legs would go up 12 blocks
And it would be 4 blocks wide
Here I have the base started

Continue building until you have the legs complete
Here you see mine

After this you could build the torso of your skin
It would still be 8 blocks long and 4 blocks wide
It would also be 12 blocks high
Continue to build that
Here we see my torso

I normally build the head after the Torso
The head is 8x8x8
Meaning 8 wide, 8 tall, 8 long
So on the back and front side of the body make it protrude 2 blocks
I have now finished the head

Now we will do the arms
they are 12 blocks tall, 4 wide and 4 long
Normally I have them 5 wide so you could tell the difference between the torso
I think it looks better but it is just a matter of preference
It will start with the torso and end with the torso
We have now finished both of the arms  have completed the tutorial!