How to rent a flat in Germany. Popular aggregators for finding accommodation

There are popular aggregators for finding accommodation - immoscout24, immowelt, eBay kleinanzeigen, wg-gesucht.de. This is not an inclusive list, but they contain most accommodation listings.

Immoscout24 has a paid subscription, the cost depends on the length of the subscription period. For example, with a 6-month subscription, the cost is 19.99 euros per month. One of the paid features is the possibility to generate Schufa directly from the app.
Schufa is your credit history showing your rating, reflecting whether there are any problems with any debts. It is required by the landlord/realtor. Schufa becomes outdated over time and needs to be updated. With a paid subscription, you don't have to pay for a new Schufa every time.
Their paid subscription renews automatically, so it is important not to forget to cancel it in time, or it will be renewed and charged off.
It is most convenient to set up filters to search for a flat and notifications will come automatically.
They are also quick to deal with fraudulent advertisements. And if there is any suspicious activity in connection with an advertisement that you have responded to, you will be notified. That's the reality, there are such ads.
Big advantage is that the interface is available in English.

On immowelt - you can order Sсhufa for 29.95 euros, from a free account. In the app you can also set up notifications and receive all new offers on time.

On kleinanzeigen, you can find a lot of ads on any topic, including rentals. They don't provide any special service.

WG-Gesucht.de is a free service for finding flats. Available languages: English, Spanish, German.

The procedure for looking for a flat through these services is about the same, you find an ad, write a request, wait for feedback in the form of an invitation to a viewing of the flat. You can take all the necessary documents to the viewing or send it by email after viewing. 

Necessary documents:  Schufa, salary slips (usually for the last three months) or a copy of the employment contract if there are no slips yet, a copy of your passport, a questionnaire with personal information (it is usually sent in advance by a realtor). 

When you go to the viewing, if the flat is interesting you give all these documents to the realtor/landlord. Then you wait until the landlord decides whether he/she is ready to rent the flat to you. If the communication was through a realtor, there may be a face-to-face meeting with the landlord. In case of a positive answer, you sign a rental agreement.

There is another way to find a flat, through special property management companies. I will write about this separately.