To cast magic equip a pouch or book, click spell scrolls in your inventory to load them into the pouch or book, then there are three ways to cast them. With /mods loaded for 'Right-Click Menu Extensions' you can load all of one scroll at one time. Right click and select 'Use all <spell name>' to load all that will fit into pouch/book.
Press number 1 to cast first spell, number 2 to cast second spell (Medium Pouch or Apprentice Magic Book), number 3 for third spell. You get the idea.
When you equip the pouch/book a spell pop-up appears on your game screen. Clicking each spell casts it during battle or clicking 'Cast All' at the bottom of the pop-up casts them all at once.
Select and Load /mods item for Autocast. Go to Menu - Game options - Autocast (on) and spells will automatically cast when each has reached its cooldown state.
Spell scrolls can be purchased from NPCs, Crafted, or dropped from some Mobs.
Spells use Magic as their requirement, the higher your magic level is the higher level spells you can cast.
Teleport Scrolls give some experience to your magic skill, but do not have a magic level requirement to use.
Fire Cracker was an event item. Other than through Rare Present it is no longer available.
For spells stats comparison your numbers may vary, depending on enemies magic reduction and your casting speed. However the order of "best" would not change. (Blizzard, Poison Rain, Meteor Storm, Holy Touch, Zeus Rage, Mark if Death, Comet Storm, Freezing Touch are high end storm. These spells deal more damage, gain more experience and are also more expensive! Most players do not use them due to scarcity and expense. Epic spells)