Step 1: You Will Need
1. A computer with the following specifications: http://support.google.com/sketchup/bin/answer.py?hl=en&answer=36208
2. Sketchup installed on the above computer. If you don't know how to install software then maybe try an easier instructable. If you don't have Sketchup installed then look here: http://www.sketchup.com/intl/en/download/
3. 30 minutes to 1 and a half hours depending on you previous knowledge of Sketchup and how quickly you work. (The first time took me about 2 hours to work out where I was going to start and the best way to construct it).
4. A mouse. Trust me - invaluable.
Step 2: Open Sketchup
Step 3: Starting the model
Firstly, once the 'creation window' has opened, click on the Rectangle/Square tool.
Click on the Origin with the rectangle tool selected.
Use the rectangle tool to create a square 50mm by 50mm.