On the left you have a list of units. If you started a new collection, this list might be empty except for one unit that was created by default for you.
If you cannot see the list of units, you might have to enable it in the View menu - → .
The check boxes in front of the unit name show which units are currently included in your practice. The number next to it is the number of words in the unit.
Add a new unit: Select where this unit belongs to, for example the collection My Spanish Lessons.
In the Unit menu, select New Unit.
Please make sure to select the right point in the tree structure of units before selecting New Unit. In our case, wanting a Sub-Unit of Collection: My Spanish Lessons it is exactly that one that needs to be selected before going ahead with New Unit.
To change the name of a unit, double click it, or right click on the unit and choose Rename Unit. You can also delete it this way (but you'll lose its contents also then).
If you don't like the arrangement of units, you can move a unit with the mouse. You can drag it onto another unit, then it will become a child of that unit.
If your units grow too big, you can choose to split them up to make learning easier in small units. 20 words for a unit is probably a good size. Right click a unit, or use → and the unit will be separated into child units. You can choose to randomize the words that are put into the new units before assigning them, or to keep them in order.