Renku is a conversation. After one poet writes a verse, the next poet must honor what went before while moving the conversation forward into new territory. Try to make your links subtle but not too subtle; never be too obvious.
An obvious link kills the forward-moving spirit of renku. An obvious link leads the mind to wander backwards and get stuck in the previous moment/image. So if the first poet mentions pimple-popping in a verse, don't follow it with a reference to dermatology! That's way too close, too backward-gazing.
The idea is to lightly touch base with the immediately preceding image (only that image, never an image from two or three verses ago!)—and move forward, always forward.
The Buddhist designers of renku had a good reason to make such a rule. Renku is a ritual for contemplating the universe and the nature of reality. To understand reality, one must not cling.< One must not get caught in a rut: a vicious cycle of repeating karma (Japanese Buddhists call this rinne; Sanskrit: samsara). One of Gautama Buddha's key insights is that everything is transient. Nothing lasts forever; nothing stays the same. So link to the previous verse, but keep the momentum forward-leaning. Move your collective poem into new territory.
For example, here's how I followed pimple-popping in the sample renku:
the remains of popped zits
on the mirror
—Nicholas M. Sola
through a telescope
deep craters
of the summer moon
—David G. Lanoue
See what I mean?
Lightly touch on what came before, but keep the conversation moving forward! If you're curious, here are linking strategies recommended by the great renku master Bashô.
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