Google SketchUp is a 3-D modelling program available for free, which makes it a tempting alternative to more expensive applications like AutoCAD. You can use it to make buildings, furniture and even circuit diagrams --- although SketchUp isn't the perfect solution for something like diagrams. Bear in mind that SketchUp models are in 3-D while circuit diagrams are represented in 2-D. Thus, it may be simpler to use another program if you have the option.
Download and install the Electrical Components model (see Resources) to your SketchUp folder. Alternatively, you can use similar or more complete collections, if available. You will use this file as a template for the components.
Run SketchUp. If you don't already have a copy on your computer, you can find the program under Resources. Open the Components model in SketchUp.
Save the model as a new file, using the "Save As..." function. This will allow you to make changes without altering the components template that you have downloaded.
Draw a large, flat surface for you to draw your circuit diagram by using the "Draw" or "Rectangle" tools.
Bring a component you need from the template to your blank diagram by using the "Copy" and "Paste" tools. If you need to orient the component differently, use the "Rotate" tool.
Use the "Draw" tool to draw in the connecting wires from one component to the next. Repeat copying, pasting and adding lines to draw the complete diagram.
Erase the component template, resize your blank diagram if needed, and add any notes or call-outs you feel are necessary to finalise the diagram.
Since you're working in an open 3-D environment, you can move and organise the templates and blank circuit diagrams in a way that will make your space the most efficient and comfortable for you. You can copy and paste the template into a blank SketchUp project, if you prefer.