Scripting a new summoning spell
On to the second part-- setting up the spell in the Spellmaking tab, and setting up an NPC to sell your new summoning spell.
Setting up the spell
Click the Spellmaking tab. Right click in the listing of spells, and select New.
Give your spell an ID and name-- let's use summon_alit.
In the Effects column, open the first dropdown menu and choose Levitate.
Set Range to Self.
Set Duration to 60.
Set Magnitude to 1.
Spell Cost is set to 21-- the same cost as the Ancestral Ghost summon. Increase or decrease as desired.

If you're saying, "I don't want to float, I want to summon something", don't worry. All in good time.
Setting up an NPC
We're going to add an NPC. Why? Well, we need a merchant to sell the new "Summon Alit" spell, and we also need a way to start off one of the scripts we'll be using. This will accomplish both.
One could just add a new start script through the Gameplay menu, but that won't work without the expansions. The method to follow should work without any expansion content.
Click the NPC tab.
Right click in the listing of NPCs and select New.
Choose a unique ID for your NPC. Our example uses _summon_alit_test_npc.
Set Class to Mage Service.
Set name, race, sex, attributes, and items as desired.
Click your NPC's Spells tab.
Click on the Spellmaking tab in the Object Window.
Left click on your new summoning spell. Hold the mouse button down, and drag it to your NPC's Spells listing.
If you've decided to not choose Mage Service for the NPC's class, you'll need to set up the sale of spells.
Click on the AI button.
Click the Services tab.
Check off the boxes labelled Spells and Spellmaking.
Click the Save button.
Click OK to save your NPC.
Place your new NPC wherever in the game you want him/her/it to be.
Now, you may have noticed that we haven't configured the NPC to do anything but sell a renamed levitation spell-- we'll get back to that later.