You can use a set of McFarland turbidity standards, which are commercially available.

https://en.wikipedia.org/wiki/McFarland_standards

By 0.5 McFarland standard

Many thanks for your reply!

Note that you still need some apparatus such as a colorimeter or other device if you want accuracy and reproducibility, although you can do it by eye (if you have the standards) if you are satisfied with approximate. You can also count cells per volume microscopically (after the right dilution of course) using a counting chamber. w