First thing, make sure your attitude is correct. You aren't going to succeed if you go into it believing that it's impossible. Spend some time visualizing your end result in first-person view to ingrain into your subconscious that this is something that can exist in reality.
After that, it's basically just feeling that she is real and there with you. Do this passively, not actively; ie, do not try to "build" her with your consciousness, just relax, know that she's there, and let the sense impressions manifest of their own accord. It's probably best to start with the basic feeling that someone else is in the room with you, this is easiest to crystallize in my experience. Again, the key is to let it happen rather than try to make it happen.
Here is a manual on how to create servitors, which are basically the same as tulpas but they have no personality and are used for practical ends rather than companionship:
http://libgen.is/book/index.php?md5=6401F10DF74BE12FD1048BA29B92FFF4
