Grow the bacteria biofilm by immersing the biofilm support/surface material in the growth medium inoculated (say 10% v/v) with pure cultures of test bacteria for at least 12 hrs. transfer support materials with adhered seed cells to fresh medium for the biofilm to grow and mature. You can perform cell counts microscopically using crystal violet staining technique or using spectrophotometry for optical density determination.  Go through these documents for details.

You can refer to the following papers (very famous for biofilm):

1. Christensen, G. D. et al. Adherence of coagulase-negative staphylococci to

plastic tissue culture plates: a quantitative model for the adherence of

staphylococci to medical devices. J. Clin. Microbiol. 22, 996–1006 (1985).

2. Merritt, J. H., Kadouri, D. E. & O’Toole, G. A. Growing and analyzing static

biofilms. Curr. Protoc. Microbiol. 1–18 (2011).

doi:10.1002/9780471729259.mc01b01s22

3. O’Toole, G. A. & Kolter, R. Initiation of biofilm formation in Pseudomonas

fluorescens WCS365 proceeds via multiple, convergent signalling pathways: A

genetic analysis. Mol. Microbiol. 28, 449–461 (1998).

Thank you Maurice Ekpenyong and Nurul Adhwa Rahman for your help

There are a few different biofilm growing strategies in this paper. Article Microbial biofilms: Biosurfactants as antibiofilm agents

Thank you Gerry Quinn