The principle is simple. Generally, meetings are held in a friendly place, like a restaurant or a cafe, on a specific day of the week. People go there to practice a language while sharing a drink.
There are several groups created on the Internet as:
Both groups require you to create a profile to access this information
Via these same groups or loquo, you can post requests for tandems. Then you offer to someone you meet on a regular basis on the principle of mutual progress in you language