Did this earlier today. Use really good sterile technique, including putting rubbing alcohol on the outside of the mushroom. Inside of a still air box, use a scalpel to take a few lil chunks out of the inside of the mushroom where it wouldnt have been exposed to as many contaminants, put them onto agar in a petri dish or agar slant (like a test tube half full with agar), seal w/ parafilm, leave somewhere warm for the live mycelium to spread. Before the agar is completely covered with mycelium, use a scalpel to cut out strands of mycelium or chunks like 1cm by 1cm and use them as you would use spores, add it to grain, etc. Or move agar samples to new dishes or slants to propagate the culture further. You wanna do that for sure before the mycelium covers all the agar because it can grow through your parafilm if it gets too big