Before you can successfully run PyPSbuilder you have to prepare working directory, which contain thermocalc, Drawpd, preferences file, thermodynamic dataset and a-x file. PyPSbuilder will work only with certain setting, like calcmode must be 1, setexcess keyword have to be present to skip question, calctatp have to be ask etc. If you are nor sure, what scripts should be set on and off, you can download sample working directory or download preferences and user files from thermocalc Richard White PT pseudosection tutorials.
In upper left pane you select phases which occurs within your field, while in lower left pane you select two phases which modal proportion should be zero. Than just click Calculate and invariant point will appear on diagram and in the list of invariant points in lower right part of window.
Similarly, you can creat univariant line, when only one phase is selected to have zero modal proportion. Click Calculate T at p or Calculate p at T according to dp/dT of reaction. Once calculated, result is added to diagram and to the list of univariant lines in upper right part of the window. Within this list you can define begin and end by selecting appropriate invariant points.
By default, PyPSbuilder use 50 steps to calculate univariant lines. You can change it in preferences dialog.
When you need to calculate some short univariant lines you can zoom into this part of pseudosection and hit calculate. Active region will be used as computational PT range. Moreover, you can manually add univariant line to simply connect two invariant points by straight line.
Selecting any univariant line in the list will highlight that reaction on diagram marked by calculated points. Note that double-click name of univariant (or invariant) line will open small window with thermocalc output, so you can always check what is going on along lines.