When I make them I use 5 petals per layer 2 layers the first being a little longer than the second
lay out the petals ( I forge mine out of 1/4 x 1 usually) make them taper to base ( MIDLE) I weld them together leaving a hole for the stem forge stem to allow petal package to slid on but not all-the way down leaving about 1 inch weld the first package
Place second - smaller package ( the differences in length allow for the greater distance the first have to travel ) weld second package - now start bending up petals like you would tighten lug-nuts on a wheel ( onside then do opposite) work around the second package until it is fairly tight.
Start first package but start in a different place and alternate one side to another being sure to tuck each petal under the next so that they lay one over the other all the way round (( do this on both sets)
Now start peeling the exposed edge back a little as a real rose might look .
or you could cut each set a petals from a sheet - I like the forged look so that it does not look too commercial - using steel I end up with about 4 pounds of rose. more like a weapon .
Here is one I made for my sister from one piece of steel No welding I started with two identical pieces one ended up being the rose leaf and the other the base.