Step 1: Open PKHeX and your save file (FireRed s.sav), then make your desired additions.
Step 2: Follow the steps shown in the video precisely, and verify that your modifications have been successfully saved to your (FireRed_s.sav) file.
P.S.: Use the Checkpoint application to create a backup; you only need to edit the FireRed_s.sav file.
PKHeX save data correct.mp4