To observe Pradosh fast, first of all you should wake up before sunrise on Trayodashi date, after that after taking bath etc. you should wear clean clothes.
Then worship Lord Shiva with Belpatra, Akshat, lamp, incense, Ganga water etc. and keep in mind that food is not consumed during this fast.
After fasting on this day, take bath again some time before sunset and wear white clothes. You should purify the place of worship with clean water or Ganga water.
Now take cow dung and prepare the pavilion with its help. Make rangoli in the mandap with the help of five different colours.
After making all the preparations for the puja, sit on the seat of Kusha facing north-east direction. Chant Lord Shiva’s mantra Om Namah Shivay and offer water to Shiva.