This is a difficult phrase.
The “just” feels like it’s a process of continuously excluding. Cutting down anything that’s not sitting as soon as it arises.
Perhaps we can reorient this “just”, into a “completely”, or “whole-heartedly”:
That the act is to continuously include into sitting. That whatever arises, knead it into your sitting.
As when you are chopping vegetables, and you dedicate your whole being to chopping vegetables: you include every part of yourself into it.