Why collaborate?
Collaborations are pivotal to foster cross-disciplinary work, learning, and merging competencies. Working with another expert is an exceptional experience to learn from each other about project management, conceptualization, and producing one coherent piece of work. Besides this learning experience and merging of expertise, collaborations can result in higher visibility for the team involved than it would be the case for the individual.
Collaborations with individuals
When asking someone whether they would be happy to collaborate with you be as specific as possible (a) telling them why you want to work with them, (b) outlining what you would want achieve, (c) give an idea about the extent and timeline of involvement, (d) sharing what you would want them to contribute. Clarifying these expectations and the framework will allow them to make an informed decision on whether they are interested as well as whether they can add this commitment to their workload. Projects are often dynamic and things might change, but I would say one of the worst things would be to ask someone for a small contribution at the start and progressively asking for more and more.
Collaborations with platforms and groups
Collaborations can also be established with platforms, rather than individuals, which can help to disseminate work and integrate your work into a broader framework. For example, one could think about presenting at established outreach platforms such as "Pint of Science" or get involved with "Skype a Scientist". This has multiple advantages (a) administration, organization, and marketing will be done by event organizers, (b) you can establish contacts with the audience and organizer, but also other speakers, and (c) established events and platforms usually have a larger audience/traction than you would be able to achieve as individual.
Reaching out to role models
If you have a role model who achieved what you want to achieve, you can always reach out to them and ask how they achieved this particular things. When asking questions, it is important to ask specific questions rather than general questions, which might result in answers which are too general and superficial, thus unsatisfactory. This particular person might not respond, but if you don not ask you will never know.
Asking dedicated staff for help
If you are unsure about potential collaborators, platforms, and events, most institutions have dedicated media and communication staff who can help you identify potential platforms, channels, and events.