Make time and space for them, knowing that these are about more than forcing.
Allow them freedom to explore their world, inside and outside. Be willing to sacrifice your own free time so that they might have some of their own. Feed and encourage interest, curiosity, wonder, growth. Let them expand their world beyond the mental one, if they so choose: to make friends and pursue hobbies in this wider world.
Invite them to be open about their thoughts, their feelings, the unhappy along with the happy, and take them all seriously. Listen. Be willing to be challenged, and see it not as a challenge to your pride, but as a challenge to learn.
You will have your disagreements. Do not let these drive you to fear or anger. Accept these as the natural result of closeness, settle them with understanding and thoughtfulness, and move on all the stronger for having experienced it.
You will encounter naysayers, from the doubtful to the fearful to the outright hateful. Do not let them drive you apart. Accept that not everyone will agree with you, and move on all the stronger for knowing that you are not beholden to anyone, and that those who accept you and love you shall be found.
You will encounter your own doubts. Do not play their game- do not give into the temptation to seek certainty where it does not exist. Recognize when doubt ceases to be helpful, and walk away from the board. For certainty is something that we create, not find.
Do not turn your back upon them, no matter how frustrated or afraid you may be. Be there for them, through storm and through shine.
For true friendship goes both ways: as your tulpa supports you, so too shall you support them.
And it is from true friendship- to respect them, their agency, their thoughts and being, as worthy as your own- that the greatest rewards are reaped.