Yeah, but it's kinda hard to explain. It's best to Youtube it. Basically you have to break old habits. Most people are used to paint programs. For instance, when drawing a line, the click the tool, then click and drag within the workspace to draw the line. Releasing the mouse button at the end of the line. Don't do that in Sketchup. Click the tool. Click the start point. Now move the mouse the direction you want the line in, don't worry about length. Once you have the direction going the right way, simply type the dimension and hit enter.
To do a rectangle, same thing. click the tool, then click in the workspace for a corner. Move the mouse to drag out a rectangle, doesn't matter how big you draw it. Then type dimensions. For a 2" by 4" rectangle you would type "2, 4" then hit enter (assuming your chosen template uses inches).
So, to make a 2x4 that was 4' long. You would do the following.
*click rectangle tool
*click in the workspace
*drag out arbitary rectangle
*type "1.75, 3.75"
*hit <enter>
*click Push/Pull tool
*click rectangle you just made
*drag up arbitrary amount
*type "4'"
*hit <enter>
Then I'd highlight the whole thing and make it a component.