If your university of choice cooperates with uni-assist, you have to register at www.uni-assist.de to upload your application documents. Once you have paid the processing fee, your certificates and other documents will be evaluated, and your grades will be converted into the German grading system.
This will take some time. Therefore, you should send your documents to uni-assist at least eight weeks before the application deadline. If you handed in your application documents in full and on time, uni-assist will forward your application to the university, or they will send you a preliminary review documentation ("Vorprüfungsdokumentation" or "VPD"), which you can then use to apply to the university directly.
For the first evaluation of your documents per application semester, there is a 75 Euro processing fee. If you want to apply to more than one university or for several study programmes, each additional application in the same application semester costs another 30 Euro.