Take a boiled potato and mash it really well. You can even run them in a blender. If you didn't mash it properly then your pooris wont puff up. So mash it well.
Now take them in a bowl, add in salt, oil and water. Mix well.
Add in wheat flour and ragi and knead to form a stiff dough.
Divide this into small balls. Take one ball and roll them into pooris.
Heat oil for deep frying. Drop poori in hot oil, press on the top using a slotted spoon. It will puff up, now flip them over and cook on other side too.