Are you from a non-EU country and do you hold a German degree? Then you can become self-employed on either a full- or part-time basis. However, you will need to change your residence permit and apply to your local immigration office (Ausländerbehörde). You can find out which legal status you can apply for and how under "What do I need". The application process requires that you write a business plan. However, we strongly recommend that you draw up a business plan for other reasons too. You can read why here.
Here you will find the answers to the following questions:
What is a business plan?
Why do you need a business plan?
What does a business plan include?
Please bear in mind that (i) the price structure, (ii) the income structures and (iii) the income and social security tax rates in Germany are different to those in your home country, and that you must also take this into account when calculating your living expenses. You should write this section in German.
You can download a German template to help you with this. The English and Spanish versions are there solely to help you with the language.