I will be your surrogate grandmother for today!
Making Persian rice is a two step process that consists of boiling followed by steaming.
Ok Dahling, so first you have to rinse the rice in warm water until the water runs clear. Then soak for at least an hour, or overnight. For a family of four I make 2 cups for one night, or 3 cups if I want to have leftovers. Make sure you use a good basmati, we use Elephant Brand #817.

Then drain the rice, and add to a large pot filled with well-salted boiling water, as if you are cooking pasta.

Boil on high until it comes to a rolling boil. After a minute or two of a good boil, add a bowlful of cold water. Yes, that's right. This forces the water to boil again, and allows the rice to grow nice and long. Stir from time to time as the rice comes to a boil.
After it boils for about 3 more minutes, taste it, it should taste firm yet partly cooked. Take it over to the sink and drain in a strainer. Run the cold water over the rice for a few moments to stop the cooking and rinse off the excess starch. Rinse the starch from the edges of the pot.

Return the empty pot to the stove, and add about a quarter cup of water and about 2--3 tablespoons of oil or butter. I added about 1.5 tbsp of mazola and 1 tbsp of butter. Also add 1-2 tsp of salt. Allow this to come to a simmer over high heat. Them remove half of this liquid and reserve.

Return the rice to the pot but don't press it down, leave it to fall in a loose heap, and sort of pile it onto itself gently.

When you start to see the steam rise to the top, you know it is time to turn down the heat to lo. Add the reserved mixture of water and oil. Stir the top gently if you like but DO NOT GO DOWN TO THE BOTTOM or you will disturb the tahdig that is working so hard to form! Leave the rice in a heap again, removing it from the edges and piling it onto the top. Cover the pot with a clean linen towel or paper towels and place the lit on tightly. The rice will now steam until it is done.

After about twenty to thirty minutes you can remove the towel. Now you can check for the tahdig. Carefully pull aside some of the rice from the bottom, but stay away from the bottom layer of rice...hopefully this will now be a nice crispy layer by now. If not, add some more oil near the bottom and turn the heat up for two or three minutes to try to encourage a crust to form.

For tonight's stew, which you can read more about
here, I didn't want any zereshk or other competing flavors in the rice. I simply dilulted some saffron that I partially crushed in my mortar and pestle. Today I used some of the water-oil mixture as I had too much, but you can just use plain boiling water.

I wish for you my dear children that your tahdig should always be beautiful and release from the pot without a hitch.

Try it, let me know how it goes.