First and foremost thing for you as an artist, no matter even if you just draw roses, is having right tools. Some pencils and brushes should do for your basic project. Also, make sure you have been well versed with these tools beforehand, and if not better try drawing something else before drawing a medieval rose, especially if you are getting paid for the drawing.
Some people recommend different starts of drawing roses. It is better to start with the base, especially if it is a three-dimensional rose flower. Drawing base first can help let the flower capture as much space as it can on your canvas, even if it is a simple piece of paper. This will also help you determine the size and girth of the other two parts above it, which are reduced more or less to petals.
You should use the broader or thicker outline, if you are just drawing rose, meaning do not plan to fill in colors in it. Thick outer edges make the flower appear flourishing and it will also increase its transparency. However, do not use too dark color on the outer edge, as it will dominate the colorless area of the rose and it might not look like a rose at the end. Just use your imagination and add to outer-line thickness as much as you think is necessary.
Finish your project with the drawing of other two dimensions, second and third. Again, use thick outline with calculating girth of the flower carefully. If you are simply drawing the flower with your pencils and do not plan to add any color to it, the drawing should reflect the picture of a true medieval rose. In fact, it will look more beautiful, if you carefully draw the outlines, than a colored flower might look. It will entirely depend on your skills and how you make use of your tools.
While drawing the picture, leave other area of the canvas, even it is a simple paper, neat and clean. This will accentuate the picture of the flower, and it will be more visible to its viewers. If you create too much mess around the flower, the flower itself will appear to dirty or will get diminished in the dirty place around it. So complete your project with a perfect stroke, neatly and beautifully.