Basically, the same way you would with one. Note an active voice construction with and without poder
alguien vende queso
alguien puede vender queso.
To use the pasivo reflejo, first obviously we remove the subject and make the main verb agree with queso (in this case, we change nothing, but if it were quesos artesanales or something, we'd get pueden).
vende(subj: queso) queso
puede(subj: queso) vender queso
Finally, we add in the se pasivo. This will generally (always? too lazy to look in the Gramática) go on the primary verb, be it modal or otherwise.
se vende queso
se puede vender queso
Note that if the object is in the singular, it can also be interpreted (and very likely would be in the case of se puede) as a se impersonal, where English often uses the impersonal you/one. The verb in that case will always be singular, regardless the object.