In order to determine your longitude, first, you have to find the time difference between local mean time and the Greenwich mean time.

We know that the meridian passing through Greenwich is universally adopted as the prime or zero meridian.

A prime meridian is a meridian (a line of longitude) in a geographic coordinate system at which longitude is defined to be 0°. For example, if you are in Greenwich, UK, then your angle of longitude is 0°.

We know that a day has 24 hours and the angle for one full rotation of Earth is 360°.

Now, divide 360°/24 hours = 15°/hour. One hour of time difference corresponds to 15° of longitude.

For example, let’s say that the local mean time at your place is 5:00 p.m. And the Greenwich mean time is 12:00 p.m. The time difference here is 5 hours.

Multiply 5 hours x 15°/hour = 75°. Your angle of longitude is 75°.

Example, if you are in Melbourne, which is 10 hours away from Greenwich mean time, then your angle of longitude is (10 hours x 15°/hour) approximately 150° E.

If you are in New York, which is 5 hours behind the Greenwich mean time, then your angle of longitude is (5 hours x 15°/hour) approximately 75° W.

Similarly, you can calculate time using longitude and vice versa. For example, If you are near Mumbai at a longitude of 73.5° E, then, 73.5°/(15°hour) = 4.90, which is 5 hours and 30 minutes. So, time at Mumbai is GMT+5:30 hours.
Note: This method of calculation will not provide the accurate angle of longitude.
Find latitude using pole star.
You can find latitude using the angle between pole star and your horizon. To find latitude, read this tip.