How do I let the player ride Pokémon with a TM?

How can I make a new TM called Ride, that can be used outside of battles.
The player switches to be on top of a Pokemon,to ride it and can jump over big ledges.

In order to make a new TM, you need to:

1.) Make sure that the move exists in the PBS file moves.txt If it doesn't, add it using the proper format
2.) Add a new item in the PBS file items.txt Again, use the prper format
3.) Go into the PBS file tm.txt and find the end of the TMs (since the file defines Move Tutor moves as well). The names of Pokemon listed there are those with which the TM is compatible. Note that NIDORANfE means Nidoran-F and NIDORANmA means Nidoran-M. All other Pokemon are in all caps.

However, you're also asking about making a move that works in the overworld. For that, I won't be able to help you.

I think the easiest way to do it honestly it just to copy all of the Bike scripts, name it "mount" or something, and then just add the appropriate sprite sheets for it (which you'll probably have to make yourself). You can even make it so that the "Mount" command shows up in the menu for specific species by default, without needing a TM. Or alternatively, you can literally just make an item (again, like the Bike) called "Saddle" or something that only works when you have the mount-able species in your party.