The most important aspect is trust and respect for each other's role.
1. Designers and product managers should take time to learn each other's role
This increases empathy and expands understanding of work tradeoffs and opportunities. The best PMs I have worked with as a designer have had some level of design ability.
2. Understand tradeoffs and opportunity cost
Good designers want to create beauty in the world, but sometimes things just need to be shipped. Great designers know the difference.
3. Let a natural process emerge
Try not to force a predefined process onto the relationship. Each individual and context is different. A natural process will emerge once trust is established and there is mutual respect for eachother's role. The soft skills really do matter.