Before adding the encounter to the actual map, you need to fiddle around in the database for a bit beforehand.
As usual, press F9 to access the database and select the tag “troops”.

click to enlarge
The troops are the different combinations of enemies the character will encounter in your game. The enemy tab is where you will import and edit your monsters whereas the troop tab is where you will select which enemies will appear during the encounters on your maps.
Let’s create a new troop of enemies for your first map. At the very bottom on the left side, you can see “change maximum”.

click to enlarge
Click ''
change maximum'' and input the number 31 (to go from 30 kind of troops to 31) and click OK.
There will now be a new line right underneath 30, the line 31. Click the line 31 to select it like so:

click to enlarge
First of all you may type the name of the encounter, although it is recommended to simply click “autoname” once you’re done as it’ll be much quicker.
Let’s make a group of enemies composed of two bats and a spider. Go ahead and select the bat in window #2 and click “Add” twice (#3).
Two bats should have appeared in window #4. Now, if you wish to do so, you may rearrange the position of the enemies. Simply click one of the enemies (you will select it and a white rectangle will appear around it) in window #4 and drag it around.
Once you’re done arranging your foes, click on the “Autoname” button to automatically name your encounter.
Note that you may click “Remove” to remove the selected monster (the monster in the white square in window #4) or “Clear” to remove all the monsters in your troop so far.
You might want to test out the difficulty of that troop in particular. If this is the case, click “Battle Test” to do so.

click to enlarge
We’ll leave the bottom half of the window for now as this is more advanced stuff. We’ll come back to it in the more advanced tutorials.
Once you’re done, click OK and you’ll be sent back to the main mapping screen.