I'd only recommend you make yourself the adult version of an imaginary friend if you have far, far too much time to spend in your own head. Those are the only parts in my life I could make room for a tulpa.
The way it went for me was forming up a little identifiable orb in my mind, the way you imagine it doesn't matter, just that you have a point that you are thinking your thoughts at. From there, do that long enough and often enough i.e. at least an hour a day, and eventually you might start getting a ball of energy you can mold into your tulpa through what is known as "forcing". Eventually, it will start talking back to you, and you'll have what is essentially a second personality in your head. Give it an avatar, fuck around in made up worlds, enjoy your time in your head. Much more detailed, and probably wrong, information is available over at https://www.tulpa.info/
But honestly, just play some games, get some friends, get a job. Even if this is probably one of the least destructive activities you can do while completely isolating yourself, I don't recommend you do it.