Self-raising flour is just plain flour mixed with baking powder so you can make it at home by mixing one teaspoon of baking powder with one cup of plain flour.
A lot of recipes call for self raising flour, which is flour with baking powder added already. You can make self raising flour at home by mixing plain flour with baking powder, but the ratio of baking powder to flour varies from recipe to recipe.
To make self-raising flour, you add 1 teaspoon of baking powder to every 150 grams of plain flour. For example, if you want to make 150 grams of self-raising flour, you need to add 1 teaspoon of baking powder to it.