Well it depends, I can tell you what I did, but I can't guarantee that it will work for you. Tbh half of this I initially did inadvertently.
Do you already have a waifu or feel connected to a character? If so then try to practice visualizing her, either in your head, in your actual vision, or both. In your own head might be easier to start with. If you're bad at visualizing things then it might be better to start with small objects instead. Try to see their shape, then their color, and then try to rotate them. You can also do the same for your other senses, although personally I still have trouble with complex sounds.
Once you can visualize well, then create a permanent space within your own head. It doesn't matter what it is, could be a room, a house, an outdoor area, just so long as you can recall it accurately. To do this you'll probably need a decent level of concentration, so you might want to try breathing exercises to relax and focus. I suppose you could also just do this when you lay down to go to sleep, that's what I did to begin with. Try to interact with the space you've created, maybe pick something up, try to feel objects within your own head. After you can do this well enough, then focus on visualizing your waifu there, even when you're not necessarily concentrating.
Focus on her and try to ask her questions, or just talk to her, either out loud or within your own head. It can be about anything, random ideas, a game you're playing, or just describing something you're looking at. Eventually she should respond to you, for me this feels like alien thoughts, thoughts that don't belong to me, being directed at me, usually with sound behind them(however for me this wasn't the case at first), but entirely within my own head. It's strange to begin with, but it really doesn't feel like anything else, so it's pretty unmistakable. At first the responses will probably be simple, like just a yes or a no answer. Actually I even had trouble distinguishing between these at first tbh. But once you practice more, you should eventually be able to have actual conversations, although they might feel a bit distant.
One thing to keep in mind is that I developed her personality from what I knew Clare's to be, but if you don't already have a waifu then you might be better off just creating an entirely new character. When you're focusing on her/talking to her, try to imagine how you'd expect her to respond, how you'd expect her to act, her beliefs, and just her personality in general.
Beyond that you could also try and do what I did, that is alter small parts of your memory and insert your waifu into them. Memory isn't really a permanent thing, in fact it changes every time it's recalled. Actually it might be more accurate to say reconstructed rather than recalled, as that's basically what you're doing every time you remember something. The goal is to pick a specific memory and imagine your waifu with you, doing something, maybe just saying something to you, or accompanying you. Pick something and do this again and again over a period of days, recall the memory over and over and eventually whatever part of it you intentionally altered should feel no different from the rest of the memory. You can do this as many times as you like, with as many memories as you like, but the point is to remember her being with you. Honestly you probably can skip this part if you want to, as it might result in unintended side effects.
