Scott Kinney:
For folks that have gone from 4th jhana to 5th, and those currently working on it; what images/feelings of expansion have been helpful in your meditation?
I have accessed relatively soft versions of the formless realms, but here is what I do, hope it can help

I don't use images for the transition, but rather an idea of transcending space. Sounds in the distance, or, rather, the potential for sounds in the distance to be heard, for example. The potential sounds come from potential objects that are also in the distance, and these objects have the potential to be cognised or experienced. But then what is distance, if the objects can be cognised? Entertaining the idea of potentially cognising infinitely distant objects opens up space, and makes barriers (such as walls) vanish in the mind's eye. Resting the mind in the infinite space between all of these things opens up 5th jhana. I guess, instead of ideas of sounds, you could use the images of infinitely distant stars or galaxies... but even the idea of a far away bus or tree is enough to open up the space, I guess, and then you extrapolate by induction. But the key for me is not in visualising the actual objects, but rather the spatial relationships between potential objects.
While we are on the subject, 6th jhana is closely related for me, I access it by letting the mind "touch" infinite space. Doing this first fills up the space with consciousness, and then consciousness itself replaces space.
7th is obtained by detaching from 5th/6th in a similar way to how you would drop piti to go from 2nd to 3rd or sukha to go from 3rd to 4th. Or the transition from A&P to Dissolution. Darkness and shrinking back in kind of describe what I go through. After it has stabilised, surprisingly, the nothingness of 7th can still be experienced, and e.g. vipassanised as "bubbles of nothingness".