Things You'll Need
Instructions
Move the TM you want to clone to the sixth slot in your inventory and go to Viridian City. Talk to the man near the southern entrance who will ask you about catching Pokemon. When he asks you a question, select no. Don't do anything or talk to anyone else in the city.
Fly to Fuschia City and walk south toward the water. When you reach the water, use Surf and head towards the Seafoam Islands. Ignore any random battles with sea Pokemon that come up during the trip. When you reach the island, stay right on the coast line still in Surf mode.
Move up and down the coast until an unusual Pokemon appears called Missingno. But Missingno is a glitch, and will either be very weak or very strong. If it's overpowered, run away. If it is weak, defeat it. The TM in your sixth slot will now have about 150 uses before running out.
Repeat for other TMs or items you want to clone. If you want to do this again, you don't have to complete the whole process. Just move a new item to the sixth location in your inventory and continue moving up and down the coastline until Missingno appears again. You can do this as many times as you'd like, so cloning master balls and rare candies will also make your life easier going forward.