I usually start by being in nature - hiking, visiting gardens, etc. Early this summer I did a lot of flower studies at Longwood Gardens. I'll often photograph what I'm most attracted to, as well as draw from life. This way I not only have a visual in my sketch book, but also a detailed image to refer back to.
Below is an example, the photo I took, as well as the sketch. I often make little notes to myself how I want the piece to look. Sometimes I'll sketch an entire piece, while other times I just have a vision of the piece and go for it.
You can view the finished piece on my website. I'll often do variations off of that drawing. Each drawing is different and each piece is set and finished different.