First of all, you need to clean the central part of the language (in particular, the median groove of the body lingual). To do this, you use the ledges on the vertical of the scraper to language. Try pulling the language as much as possible and put the scraper as far as possible to the back of the mouth on the tongue. Then, slide the scraper on the tongue (from the back towards the apex, passing through the central part of the language).
Don't forget to clean the scraper to language after each movement.
Clean the side parts of the tongue with the smooth side of the scraper. Again, don't forget to clean the scraper after each movement.
Finally, rinse the mouth well (possibly several times).
It may be that you have a reflex to retch the first time you use a scraper to language. This feeling will disappear little by little.