One point perspective as the name suggests is using a single vanishing point to create objects using perspective. As I said in a previous post this vanishing point is located on the horizon line. Before you start to draw anything you will need to decide where on this horizon line you want to place your vanishing point as this will determine from what angle objects apear to be viewed from in your drawing.
Below I will show you how to draw a simple cube using one point perspective, to keep things simple I will have the vanishing point directly in front of the viewer.
In the image above I have added the first few lines for the base of the cube (in red)on to of a grid of lines (in grey) coming out from the vanishing point (black dot). While I have started with the base of the cube this is not compulsory you could start with the front or the back if you wanted to. Take note of how the sides of the red outlined square follow the lines of the perspective grid, it is by doing this that adds perspective to your drawings.
For the next step we will add the front of the cube. when drawing this side there are no guides to help you in terms of perspective except for the base of the cube, just try to make the sides as straight as you can and give it whatever hight you want.
In this image I have added some more lines to the perspective grid I was using from top two corners of the front of the cube to the vanishing point. What this does is give us a guide for drawing the top of the cube just like the perspective lines were used as a guide for the base of the cube.
Using the perspective lines that were just drawn we can now finish the cube first by joining the far corners of the base of the cube with the perspective lines connected to the top front corners of the cube then joining that with the a horizontal line. From here it is easy to add the remaining lines to finish the cube.
I recommend that you practice this as it will become very useful later when you start trying to draw more complex objects in perspective.