This is for you, if you want to re-create the image I'm using for my pencil crayon tutorial (which I will upload very soon). For this outline, I used Micron ink pens. You can usually buy them in most art stores. Micron ink pens do not bleed when you color over them with pencil crayons. Other options are Staedlar ink pens- but they tend not to be as smooth, and you have to let them dry longer. Another option is Copic outliners, but they are more expensive and tend to be harder to find.
I start by creating a fairly detailed sketch with pencils. I make sure that I fix all my mistakes in pencil stage so that I know what I am going to outline in pen. Then, I outline everything with a 0.005 thickness pen. After that, I begin picking out detail with larger pens. This is not necessary, but varying your line thickness does create more interest. There is really no rule (that I know of) for where you should thicken your lines, but I generally use it to smooth out shaky edges and create smoother points. Also, often where lines converge, the ink will be thicker to 'soften the edges'. After outlining, I let it dry for a couple minutes before erasing the pencil lines underneath and then coloring. Make sure when you erase, you do it slowly - I've ripped and ruined my pictures too many times because of that.

You do not have to outline your drawings in pen - but that is the style that I use, and that you will need for this tutorial. However, you can create some beautiful and very different pieces of art without doing so, and it is worth experimenting.