You only need a simple image editor such as MS Paint, I use MS Paint.
For an alternative, see this Instructable
Step 1: The shrink
Use the rectangular Select tool to select the area you wish to pixellate.
Look to the lower right of the Paint window, the dimensions of the area will be shown as you drag the mouse. Select an area that is a whole multiple of 10 (see step 3 for more on this).
From this point onwards don't touch the image or the right hand toolbar with your cursor - only use the menus.
From the Image menu, choose Stretch/Skew... (CTRL-W)
Reduce the size of the area both horizontally and vertically. Here I am making the area 10 x smaller by selecting 10%.
In doing this shrink there will be 100 times fewer pixels in the selection.