Trimble Sketchup is a useful intermediate level tool for the creation of 3D models for mockups and 3D printing.
Export the model to somewhere that can be accessed later on, and make sure to save it as the OBJ (*.obj) type using the Export Type drop-down menu at the bottom of the window. If you are unable to save as a OBJ, save it as a COLLADA (*.dae) and convert it using this website to an OBJ or STL.
Next, open up the MakerWare software. From here, click the large [+] Add button and find your exported 3D model. If the software asks if you want to put object on platform, click Move to Platform.
To set the dimensions of the 3D model to be printed, click Scale on the right hand side of the window twice and select the model. To set the size in inches, type the dimensions in and click Inches -> mm. This will change the inches that you have typed into the boxes into their equivalent millimeter measurement.
Once the 3D model is how you want it, click the large Make button near the top of the screen.
Once the button is clicked, a window will pop up. To save it to your SD card in order to print away from the computer, choose Export to a File. If your computer is connected directly to the printer via USB, you can choose to Make It Now, but make sure the computer does not fall asleep while the object is printing. Resolution can be changed to Standard or Low to simplify the model in order to have it printed faster. Raft will build a small base around the bottom of the model, and Supports will build supporting beams for the model.
From here, click Export!, and either allow the model to print from the computer or save the model to your SD card.
To print from the SD card, plug the card in to the slot on top of the screen of the printer.
Press the M button to choose Build from SD, then select your model with the M and allow it to print.