The roses should be fresh and used within a few hours. Hence it is better to string roses when they are fresh. Of course for stringing of jasmine, roses and jasmine garland videos are there in Rangolisansdots my video Channel. If the explanation and images shared here are not sufficient, the videos may help.
The flower shown in the image is kakada that is some what similar to jasmine but has almost no fragrance
To have a recap - the first image shows to flowers placed one above the other with stalks over lapping and with the string at the bottom
The next image shows the flowers being held with the left thumb, with the string going around the stalks so as to hold them.
The third image shows the beginning of the formation of the knot using the index finger and middle finger and rolling them over the string first along the string then with the middle finger supporting the string roll the wrist around the string to get the position shown in the fourth image. This is the most difficult part in this process as it involves rotating the fingers and wrist in different planes.
The fifth image shows the formation of the ring which is inserted on the top of the flower as shown in the sixth image so that the the ring is over the overlapping stalks.
The final image shows the string being pulled so that a knot forms around the overlapping portion of the stalks. We can see flower vendors doing this quite fast and mechanically concentrating on some thing else while stringing flowers. That is practice and experience for us !