Reusable face masks, filter??

[Crazydaisy11]

A friend of a friend is making face masks so I bought one.
Its cotton with a cotton flannel layer and with a filter pocket. Probably a really stupid question but do I need to slide something into the empty pocket to make a filter? Then throw away, wash face mask and put a new filter in whenever I use it?

I've seen people using pieces of kitchen roll as a filter or one of those filters you put in a vacuum cleaner

Do you mean the actual Hoover bag, cut a bit and slide it in?

No i mean filter mesh like this