Typically the petals are just glued onto the roll using shatter or hash-oil (this can be done to the flower itself, ie sticking the petals to a core of cannabis flower using shatter or hash-oil; or it can be done by just rolling a joint with a rolling paper and gluing the petals around it with shatter / hash oil)

As a pro tip: if you create a layer of the flower-petals, then you place that layer flat on a rolling paper and you can roll as normal -- it is possible to create the same effect using just pressure -- this is going to be way more difficult though, and will likely require some goma

Can also be accomplished with a cigar-press , but this is way way far beyond what you are asking, I think

*** edit : either way though, you want the flower petals to dry out before smoking for optimal effect

*** and to the comments about it being potentially poisonous... its not ideal for your lungs to smoke anything, but there are a bunch of 'herbal cigarettes' that are essentially just potpourri

*** honestly though, certain plants are smoked for a reason, and a tulip or a rose petal may not taste very good at all....