First heat dhokla steamer. Then take a small portion from potato mixture. Then roll it. Add salt and soda in required idli batter.

Now take a glass and grease with oil now put middle of this roll and on-the-spot fill idli batter around it.

See in the picture. Now put this glass in dhokla steamer and steam for 10 minutes. After 10 minutes cool down it and and unmould with the knife and cut and serve with green chutney.