Ok, so if you want a VC save file with YOSHIRA Mew and YOU DON'T CARE ABOUT LEGALITY, you need:
A modded 3DS with VC game installed
A PC with PKHeX
If you want to completely replace your save file with the original SAV dumped from the original cartridge they used in 1999, full of all the YOSHIRA Mews, you have to:
Use Checkpoint to dump a copy of your actual save file (you can follow this guide).
Insert your SD card into PC and find /3ds/Checkpoint/saves/(your game)/(your save slot)
Make a backup of (your save slot) folder in a safe place.
Download the YOSHIRA SAV
Open YOSHIRA.sav with PKHeX and export it with the name "sav.dat"
Replace "sav.dat" file in the (your save slot) folder on your SD card
Reinsert your SD card into your 3DS
Restore the save file with Checkpoint
If instead you want a single YOSHIRA Mew in your actual save file, you have to:
Use Checkpoint to dump a copy of your actual save file (you can follow this guide).
Insert your SD card into PC and find /3ds/Checkpoint/saves/(your game)/(your save slot)
Make a backup of (your save slot) folder in a safe place.
Download your single Mew
Open your original "sav.dat" file with PKHeX
Drag and drop the Mew pk1 file in a Box
Export your save file (always with the name "sav.dat") and replace the original one in the (your save slot) folder on your SD card
Reinsert your SD card into your 3DS
Restore the save file with Checkpoint
OBV YOU COULDN'T TRANSFER THIS MEW IN GEN 7/8