I use Great White myco in all my plants, marijuana and otherwise. That stuff is AWESOME; it's the best root building fungal product on the market.
Like everyone else said, just dust some in the hole before you plant your seedling, and it will do the rest. It's a mix of fungal spores that, when in soil and nearby roots, forms a symbiotic relationship with the plant that encourages root growth and aids the root network in absorption of nutrients. Roots are the most important part of your plant's health, and this stuff works wonders on them!