There are two ways to block in a practitioner's time for things like covering another practitioner etc. In iconpractice these are referred to as "Annotations". The quickest way to enter one or two annotations for a single practitioner is to click in time slot which will be the start of the annotation and select "Block Out/In":
Then enter the heading or title of the annotation, select a background color, select "Make Available", set the end time and click Save.
Entering Multiple Annotations
Multiple or recurring annotations are best entered via the Settings section. Visit Settings->Diary Settings->Annotations. The process for entering annotations on that screening is described in this article here.
Video Overview
This video describes how to enter an annotation via the diary starting at the 3 minute mark: