It sounds like the process could be done in a way that is similar to when you invade someone’s dreams, although easier. I don’t have any exact methods for this but I have used daydreaming and intent focus to place thoughts in someone elses mind. Without using an actual spell though, and just throwing thoughts someone elses way, it could end up taking much longer than expected because that method is sort of half assed but works great if you don’t need results right away.
If results are needed faster, I suggest doing a candle lit meditation while focusing on the person whose thoughts you wish to add to. Visualize the person and the thoughts you wish to place, and then see them receiving the thoughts and acting upon them or being inspired by them. Play it out in your mind like a meditative movie and pour all of your focus into it. It works the same as a spell but requires no ingredients other than your mind and imagination, and a candle to help send the intended images by way of air (if desired).
It can be extremely effective if you devote the right amount of focus to the target individual.