It can be very fiddly but it is super-powerful once you reach comfort with the UI.
Building upon Praetor’s great post above, let me suggest this approach:
- make a quick first draft of your UI.
- in the Unity game window change the shape / dimensions of your Game window (you can even make your own resolutions)
- see what happens to your UI in this new shape window
- iterate and fix your anchors and scalings and whatnot
Make sure you do not overlook the Canvas Scaler settings. Personally I (almost) always use Scale with Screen Size and Match Width Or Height, with 100% height because it seems the most useful. I also NEVER change the 800x600, which lets me copy/paste prefabs between any of my projects trivially.