I am currently growing 6 plants and I did mykos in 3 of them. The three with mykos are noticeably healthier. They look nicer and even seem to have more growth and bud weight on them so far.
I have 18 plants, and I used about 5 mini bags of Extreme gardening mycos, each bag is supposed to handle 6 plants, but I wanted more density for faster growth rate. I used about 1/4th a bag per plant didn't just put it on top, I layered it and mixed it up in each layer while filling the pots with soil, also mixed in perlite. Soil formula I'm using is Formula 707.
For plants that are already established you can use a drill bit on the sides of the pot at an angle towards the root mass, once you have a hole you can fill it up with mycos and then a little soil on top... I did about 3 auger sites around the plants angled towards the root mass, but not directly into the root mass... that way the mycos would grow Toward the root mass not out from the root mass... Buds are growing pretty big on 3 of the pre-established plants, while the new plants will have mycos all throughout the soil from top to bottom. You could also use the powdered mycos from extreme gardening that could be used for hydro or pre-established plants just mix with water and water with it every time you water for about 2-3 weeks, that way you have a nice dense mycos network.
Don't over water your plants, that will kill the plant and the mycos.