To start inscribing, go to Inscription icon. It will open the Inscription menu. Click on the unit you wish to inscribe. I am using the Royal Knights in my example today.
Now it opens the inscription menu for the Royal Knight. The insciptions will be visible under the properties. Note that the inscription is not yet activated. Check out the basic attributes of this unit without any inscriptions.
In this example, I use thrifty inscription. Note the 4 SETS stated beside it which means this inscription in order for it to get activated, you must inscribe four pieces of them. Thrifty inscription will allow less resouces needed to recruit a unit, and each shape has an additional attribute. Note also the number of stars on each of them, and the different colours some inscriptions may have.
Inscriptions fit into slots like lock and key. You can fit in any shape as long as it meets the number of sets needed to activate them. In this example, 4 sets are already placed into respective slots and note that there is no more 'not yet activated' phrase, but instead the attribute of the inscription is stated. Check out the attributes of the units are now increased (in green). So what to do with the empty slots? You could still fill them up with inscriptions that can be activated with just 2 SETS. Get it? Look at the following example:
You could notice that there are 4 + 2 inscriptions. Perhaps you could try inscribing the 2+2+2 inscriptions.
Good luck.