I don't have any advice as I haven't done the multiclassing yet. Either you try to find some videos to give you ideas on YouTube or you just try it yourself. The good thing is, respeccing got unlocked very early (you have to unlock it, however), and it's practically free. So you can do trial and error. I respec my character a couple of times already when I want to do a new build (for example after I got a new or interesting gear). Things you cannot change are: your race and background.