DG hasn’t publicly commented on the process. Astral projection would indeed be a credible method for Jamie to visit via his dreams. Precognition would be another method. This would make a great question to ask DG directly.
Let me add considering the abilities we have witnessed in both Jemmy and Mandy DG has some interesting explanations coming our way. These gifts are areas I wish DG had started expanding throughout the series instead of leaving to the end to tie up.
