Start with an .STL file of an object you want to print. Thingiverse is a great resource for different things to print.
Open Slic3r, and import the .STL file. Change any settings as required.
Generate the GCODE output using Slic3r.
Open Printrun, and import this GCODE file.
Connect to the printer, and press the 'Print' button in Printrun.
The heated bed and extruder should begin to heat up. After they've reached the correct temperature, the print should begin.
Watch to ensure that the first layer sticks correctly to the print surface. Depending on how well the Z offset is calibrated, the nozzle may be too high or too low. The extruded plastic should be squished down between the nozzle and the bed - it should be flat and wide, not thin and cylindrical. If the height is wrong, abort the print, adjust the Z offset (as per the instructions on the calibration page) and try again.