The Sun Method. This method involves put the flowers into a bowl of spring or well water, and putting it in the sun during a good sunny day.
The boiling method. Simply put, the flowers are boiled in a saucepan.
Of course, there are specific procedures for each remedy, but those are the two ways of doing it. It should also be noted that conditions have to be exactly right in order to obtain the proper result. The time of year, the plant in question, the weather, etc, all should be perfect.
Anyone who makes them in their backyard without first studying how should be considered as suspect. (Of course, they might be working on guidance, but there are many who do not seem to be!)
For those who want more information, the book The Bach Flower Remedies - Illustrations and Preparations by Nora Weeks and Victor Bullen contains full details on making the remedies.