Collaborating (online) with researchers from other institutes/countries
I have found that it is very difficult to start collaborating with people who are totally new to you. Some good ways to find people to collaborate with are:


Tapping into your PhD/PostDoc advisor's existing contact network
Using your advisor's status/fame to get new collaborators (i.e., the advisor handles the initial contact)
Giving talks in conferences and getting people excited about your work.
Talking to people in conferences and finding some common interests for new projects.


I write this as a PhD student with experience in collaborating within physics/chemistry communities.