Using this method, you can find Mew in the game Pokemon Red / Blue or Yellow. This may require completing the game from the beginning, depending on how far you've come. But isn't it worth catching such a rare Pokémon?
Steps
Method 1 of 4: Glitch with Gumbler and Youngster

Step 1. Restart your saved game if you've gone too far
Using this glitch will require you to fight Gumbler and Youngster.

Step 2. Go through the underpass from Lavender town to Celadon City, face Gambler and immediately press the Start button
Instead of pausing the game, you will thus enter the Game Menu.

Step 3. Fly to Cerulean City
Gumbler will notice you after choosing the Fly menu item and you will hear the music of the battle, but in any case, you will already be far along the way to Cerulean City. The Start button will become inactive, but shortly thereafter, it will work again.

Step 4. Find Youngster in the bushes just behind the Nugget Bridge
This is the second Youngster you will come face to face. The same was above "Lass". Now it is very easy to defeat him, since of the Pokémon he only has Slowpoke.

Step 5. Defeat the trainer and immediately fly back to Lavender town
Your Start button should now work again.

Step 6. Go to the left exit of the city
The pause menu will automatically pop up.

Step 7. Unpause the game to start the battle
Be careful, because Mew can only be level 7!

Step 8. Catch Mew with the Master Ball or weaken it with your attacks
Or, if the Pokémon is weak enough, try catching it with the Poké Ball.
Method 2 of 4: Glitch with Trainer and Youngster

Step 1. Restart your saved game if you've gone too far
Using this glitch will require you to fight the trainer hiding in the grass and Youngster (both in Cerulean).

Step 2. Catch Abra (unless you have another Pokémon that knows the Teleport skill)
In the game Pokémon Blue / Red, you can find him on Route 24 and 25, as well as on Route 5 (you need Pokémon to put him to sleep).

Step 3. Go north of Cerulean and stand in the same spot you see in the picture or video

Step 4. Save the game

Step 5. Go ahead and immediately press the Start button

Step 6. Select Pokémon Abra and his Teleport skill
The trainer will see you, but either way, you will already be at the Cerulean Pokémon Center.

Step 7. Approach Youngster (who will lead you to Bill); this is the third Youngster
Avoid any fights up to this point, and stop at some distance from him (to allow him to move).

Step 8. Win the battle with the trainer (who only has Slowpoke available) and use Teleport again
You will be transported back to the Pokémon Center.

Step 9. Go north again
At some point, a menu will pop up. Get out of it and the battle with Mew will begin!
Method 3 of 4: Using Glitch to Catch Any Pokémon

Step 1. Improve your fishing skill
Now that you know how tricks work, use them to catch other Pokémon. Fighting Youngster will always lead to a meeting with Mew, because his Slowpoke has a Special skill of 21 (15 Hex). But if you fight someone else before returning to Route 8, the Slowpoke Pokémon will change to another. For example, some say that you will regret choosing not Bulbasaur as your starting Pokémon. The Bulbasaur Hex number is 99, which corresponds to 153 in Dec. You can now fight the wild Pokémon Chansey, which has a Special skill of 153, but you can only find him at Level 64 in the Unknown Dungeon, and this happens quite rarely. Worst of all, the chance of finding a Pokémon with the correct Special is one in sixteen. Fortunately, there is a solution: the Transform skill of Pokémon Ditto, which copies the statistics of the enemy, will help you with this. This means that you can catch any Pokémon in the game by following the instructions in this section.

Step 2. Catch or upgrade Pokémon with the appropriate Special skill (For Bulbasaur it is 153)
You can find a complete list of values required for each Pokémon at the bottom of the page.

Step 3. Fly away avoiding the battle with Gumbler

Step 4. Engage in battle with any trainer who can see you from afar and come up to you (this is necessary to unlock the Start button and get into the battle on Route 8)

Step 5. Find a wild Ditto and let him Transform your Pokémon with the appropriate Special

Step 6. Defeat Ditto (or simply run away) and immediately fly to Lavender, without engaging in a duel along the way

Step 7. Move to Route 8, where Bulbasaur (or someone else) will appear

Step 8. Please note that since only the lesser of the two Special skills is taken into account, you can increase it to a value above 256 and use it to get the same Pokémon
This is important because there are several Pokémon, like Kangaskhan, whose ID values are lower than the minimum Special of any of your Pokémon. In the case of Kangaskhan, you can find her with a Special skill of 258 (2 + 256).
Also note that during this trick, any trainers you fight after Gambler will be marked as "used". Make sure you have enough unused trainers so you can catch all the Pokémon you want! Fortunately, Gumbler can be used to activate this trick an unlimited number of times
Method 4 of 4: Glitch with Gumbler and Biker

Step 1. Restart your saved game if you've gone too far
Using this glitch will require you to fight Gumbler and Biker.
2 Go through the underground passage from Lavender to Celadon, face Gambler and immediately press the Start button Turn right as soon as you see water in front of you. The Biker you need will be facing the grass. Defeat him. Fly into Lavender Town, and then head left to Route 8. The Start button menu will automatically pop up. Get out of it. Catch Mew with the Master Ball, or weaken him and then catch him in the Pokeball.
Step 3. Fly to Fuchsia city (after that the Start button will be temporarily disabled)
Step 4. Take Bicycle Road and then head up
Step 5. When the bike slows down, continue upward
Step 6. After that, walk until you come across grass
Step 7. Approach the Biker
Step 8. After fighting him, your Start button will work again
Step 9. After that, your Mew will appear
Advice
Warnings