The coordinate of your base station must be in the coordinate system WGS84 and they must be in meters. If you need help converting your coordinate system to WGS84 you can find help here.
After converting your coordinates to WGS84 click on the options tab on rtklib highlighted below.
Navigate to the Positions tab and select the Lat/Lon/Height (deg/m) option from the dropdown box highlighted in the picture below.
Then enter your coordinates in the text boxes underneath the drop down box.
Now press OK on the options tab and then press Execute shown below.
After Processing finishes Press Plot.
After selecting your base coordinate you can follow through to Clipping the Observation File.