If you haven’t read my Introduction To Thoughtforms I suggest reading that before this, this is a low-tier method and is really slow to do, but it can work if you need to use this method.
Thoughtforms can be created through repetition of thoughts to something, this method can take hours or even months depending on how skilled you are with it. But of course, this method is inefficient but it is the best method to make long lasting Thoughtforms as well as sentient ones.
The first part of this method is developing the idea that the thoughtform is there, for such you should constantly talk to the thoughtform as if it already exists, as if it’s a physical person that is next to you, either out loud or in your head will work.
If you are wondering how this works, it gives your mind the idea that there is a person that you are really talking to, and due to the absence of it, it creates said person as a thoughtform.
The next step is the form of the thoughtform, I suggest doing this in your soul landscape if possible but in a wonderland area is also okay, (Search up Tulpa Wonderland). You should visualize the thoughtforms body/form completely, and as you do such treat it like art. Do not just rush it, get everything right that you can else it will be deformed. The next part be sure to feel the body of the thoughtform, how the surface of it feels, texture, hair/fur, etc.
If all goes well you should eventually have a thoughtform, it will take a while for you to be able to communicate correctly with said thoughtform but you most definitely will have one. Be sure to make sure it knows english and such, but as it is created by you it should, but this thoughtform type can’t do much, it can technically leave your body on the thought-plane (Not the astral, different), but it can not do things energetically an such, but there is a way around that..
The way around such is forming a energetic construct body for it that it is allowed to interface with and develop on its own, it's best to give the thoughtform the knowledge you have on energy manipulation and such for this as well.