I've found over my many years of shroom growing the best way to make a viable spore print you can store for years is this.
Go out and buy some gaze bandages, I like 4"x 4" but it really depends on the size of the mushroom cap.
Now, while wearing gloves as sterility is key to keeping a viable print. Remove the gaze and keep the the sterile waxed paper it came in.
Using the inner waxed side of the paper, place it on a flat clean surface. Put your desired mushroom cap on the paper and place a glass over this. We want the humidity to rise to encourage the cap to drop more spores.
Leave the glass over the cap for a few hours. Then remove the glass, the cap while being careful not to disturb your work to much.
You now have a sterile spore print that can be used right away or stored for many years.
To store your print take the top half of the bandage paper place it waxed side down on the printed side of the other paper. Use scotch tape to seal the edges then place in a emptied and sterilized film canister seal this with electrical tape and put in the fridge.
If anyone wants me to explain how to make a syringe from that print, just ask.
Hope it helps.