In a kadhai/wok/thick bottomed pan add the seeds and on low flame gently toast them. Keep stirring constantly the colour will change. This is best done on stovetop and not in an oven since the sesame seeds are small they burn easily.
Cool the seeds completely before you start processing. I prefer the mixer/grinder to process the seeds. Add the cooled seeds to a suitable jar run the mixer till the seeds resemble a crumbly mixture.
To the crumbly mixture add 3 tablespoon oil. Run the mixer again, you will have to add the oil 2-3 times. Scrape the sides and bottom,
Add 1-2 teaspoons only every time. Remember to scrape the bottom and the sides of the jar before adding.
In case you do not want to add more oil it's fine. Your tahini will not be smooth and you may find it difficult to pour it out.