Welcome to the forum Florentin
I don't have personal experience germinating R. myrtifolium because it is usually too early to collect seeds when visiting the Carpathian Mts., unfortunatelly.
But taking into consideration the few Rhodos I had germinate, the process it's pretty straightforward.
Collect the capsules, let dry at room temp., they should open and release the seeds; keep them in a glassine/paper envelope (a glass jar will do well) and sow in early spring.
The sowing must be done on top of the substrate (they require light for germination); I won't give you substrates recipes that I know you won't be able to follow by lack of 'ingredients'. Use a potting mix with added vermiculite/perlite/fine gravel or something similar. Easiest is to enclose the pot in a large plastic bag until the germination starts and then open it gradually to allow more air for the seedlings and check on the humidity level.
Very important - good luck
