STEPS
TOOLS
Get your 2 pieces of line
Place one under the other.
Do the same at the top.
Pull tight.  I left these simple because I thought the pictures were pretty self explanatory. There are other ways to tie this, with a loop in one end and going around it with the other.
 Rope, line, string, etc...

How to tie a square knot

A simple effective knot. May slip when wet under tension.