First look for the planet that has the highest number of degrees. It is henceforth referred to as the atmakaraka. 

Find the atmakaraka in the navaamsh chart. It is henceforth referred to as the karakamsa.

Look at one house before the location of the karakamsa. In other words, you should look at 12th house from the location of the karakamsa.

The lord of this house is the isht of the person.

For example:

See this Navaamsh chart:

The planet with the highest degree is Venus - 22:57:41.

Find Venus' position in ‘Navaamsh’ chart . In the first example, it is sitting in the 1st sign - hence Aries. Now analyze the twelfth house from third , which in this case is the 2nd house with the 12 sign.
As no planet is placed in Pisces, we take the lord of Pisces - Jupiter to base the isht dev on. The person should worship Vishnu as much as possible.

Here's another example: 

The planet with the highest degree is Moon, which is located in the 4th house, whose ruler is Cancer. There's a planet in the third house- house of Gemini - coincidentally the planet in the house is the same as the ruler of the house - Mercury, based on which the isht can be derived. The person may either worship Ganesha, Vishnu, or bhagwan Buddh.

Note:

Ketu can never be regarded as the atmakaraka, so if Rahu and Ketu have the highest degrees, look at the placement of Rahu in navamsh.

If there are more planets than one in the 12th house from the atmakaraka, the strongest planet needs to considered to derive at the isht devata.

If Sun is associated with Ketu then Śiva worship is indicated.

If Moon is associated with Ketu then Gouri worship is indicated.

If Venus is associated with Ketu then Lakshmī worship is indicated.

If Mars is associated with Ketu then Skanda worship is indicated.

For both Mercury and Saturn's association with Ketu, Vishu/Krishna worship is indicated.

If Jupiter is associated with Ketu then Sāmbaśiva worship is indicated.

If Rāhu is associated with Ketu then Tāmasī or Durgā worship is indicated.

If Ketu alone associates then Ganeśha or Skanda worship is indicated