First, create a text file with PlantUML commands, like this example called sequenceDiagram.txt:
Alice -> Bob: test
Then, run (or have your software call) PlantUML, using sequenceDiagram.txt as input. The output is an image, which either appears in the other software, or is written to an image file on disk. For example:
java -jar plantuml.jar sequenceDiagram.txt
This outputs your sequence diagram to a file called sequenceDiagram.png.