What should be the correct order to learn Hindustani music?

First of all

I am giving you a step by step method. This has short term targets, which add up to a long journey. Doing riyaaz for 1–2 hours is hard, but doing it the correct way is more important.

1. Select a few sargams with fluctuations.
2. Categorize them from simpler to harder.
3. Start from basic. Sargam of shuddha swaras. (E.g. Sa-Ga, Re-Ma; Sa-Ma, Re-Pa)
4. Alternate your speed for each sargam. Start with slowest possible speed with which you can sing melodiously. The low speed will give you time to judge and determine your next note.
5. Gradually increase your speed for the same sargam. Do this until you are very much COMFORTABLE with the same sargam in any speed.
6. Just a teeny bit of modification. Change one swara of the sargam to komal swara. Only one. Repeat steps 4 and 5.
7. Undo this change.Change a different swara to komal /teevra. Repeat.
8. There will come a time, when you will be confident to sing with more than 1 modifications in the original sargam. Do it. Mind, we are STILL on the simplest, first sargam.
9. Eventually, move on to other sargams. Follow same pattern.
10. There willl come a time, when you will be happy with your sargams. Move on to Behelava. It is a string of notes which are very near to each other sung with continuous meends. Follow the same for Behalava as you did for sargam.
11. Yay!!! You are there!!!!

Some additional tips :

Do Pranayaam to increase vocal strength.

Avoid singing false notes if you cant reach higher notes. Dont strain your voice

Then u can learn more category of  hindustani Classical music like Raags,chota & Bada Kheyal,Thumri,Tappa,Tarana......step by step