There are a number of ways to do so:
Donations to the state budget fund for relief can be made through these details (click to enlarge):
The Association of Croatian American Professionals Foundation, together with Crodiaspora and the Canadian Croatian Chamber of Commerce has set up a gofundme account, which is supported by many diaspora groups around the world. See here
The Croatian branch of Caritas can be donated to here (in Croatian)
The Croatian Red Cross can be donated to here.