Can I use German flour Type 405 for a recipe that needs self-rising flour? I'm totally new to baking/using flour in general. I'm wondering if Type 405 is enough for a recipe that specifies "self-rising flour".


  Germany's Flour Type 405 is equivalent to pastry flour. Pastry flour is made from soft wheat and has a gluten content of 8-10%. It is soft and ivory in color. Because of its low gluten content, it is best used for baked goods that should have a soft consistency yet still needs some structure, such as muffins, buscuits, pie crust, tart dough, cookies, some sweet yeast doughs, etc.
  
  Pastry flour is available in the U.S. in health food stores, specialty stores, and mail-order businesses. To make a flour with the same gluten content as pastry flour, combine 1 1/3 cups (185g) all-purpose flour and 2/3 cup (90g) cake flour (which is available in most markets and has a gluten content of 6-8%).
  
  Source


FWIW, I'm planning to make some home-made munchkins. The recipe doesn't need an oven but is deep fried.

If it is not suitable is there anything I can do/add to make this kind of flour be a suitable substitute for self-rising flour?
The keyword here is “self-rising” - which describes a mix of flour and baking powder.

So if you are going to use 405 instead, which would probably be a good—enough choice (especially as it’s “the standard flour” in German supermarkets), you have to add baking powder. As a rule of thumb, use one pouch (assuming you are in Germany, where that’s the standard unit) per 500g of flour. If your recipe calls for self-raising plus baking powder or baking soda, use the extra amount as well.

I personally prefer he German 550 flour as my go-to type, which is closer to what other countries call “all-purpose”, but it may or may not be available at your store. Unless you are making bread and similar goods, 405 will most likely be ok.