In this guide, you will come to know how to turn your Sim into a Spellcaster in The Sims 4. You have to follow the step-by-step instructions in this guide to do so.
You will first need to travel to the Glimmer Brook neighborhood, which is unlocked with the realm of the magic expansion pack. You have to go to the western lot of Glimmer Brook, watch and follow the path towards the northwest side when you get there. You will find a magical portal aschway there, as shown in the image below.

Now you have to use it to travel to the magic realm. When you get there, you need to go towards the north side, where you will find the main building.
Now you have to travel through the three different sages; Morgan Ember, the sage of untamed magic; Simeon Silver’s water, the sage of practical magic and Elphaba, the sage of mischief magic. You have to talk with them and ask how to use magic.

When you ask them, they will give you a task, and you need to collect seven magical motes in that task. The sage will cast a spell on you so that you can easily see them, and this magic lasts for eight hours. To complete the task, you need to walk around the magic realm area, and wherever you find the motes, you need to collect them.

Once you have all seven, you have to go back to the sage and turn it to sage. Now they will agree with you that you were born ready and transform your sim into a witch or wizard, and right after that, you will start learning and training new spells from them, as shown in the image below.
