How do you make a mini itx build?

I don't know what parts to pick for mini itx build.In fact,I don't even know what a mini itx build is.You don't really get an answer when you google the question.Like I know it's small,but not about the itx part.I don't know how much space it takes up or how hard it is to find/make one.Any help?

Edit:Type in simple words since I don't understand a lot about computers.