Select the feature that would like to create a buffer for & ensure the snap tool is active. Yellow is active white is inactive.
Next create a guide to tell the software where the buffer needs to be created by drawing a line. Click on draw and select line.
Move your cursor to your starting point and you will see that it is magnetised to the edge of the feature. Single click to start drawing your feature.
Move your cursor around the feature and it will snap to the edge. Double click to finish and click on finish.
Select the line and then click on buffer.
Input your buffer settings. Side one is the exterior, side two is the interior. Input the width needed for the buffer. Select your settings for rounded corners and subtraction and click on OK.
Your partial buffer will be created.