The short answer is “Just do it”, but that often doesn’t work. The old advice about making a list though, that does. The trouble is that it’s easy to get caught in a cycle of making lists of lists, and that tends to produce even more lists.
Of course, making a list is ‘getting something done’, in itself. Once you’ve done that (made your list), move on to do the first thing on the list, without analysis, deviation or procrastination (“I’ll just get a cup of coffee first…”, “Maybe if I wash the cat/do the ironing/go shopping I’ll be able to get more done later…”, etc. [I’ve done all of them, often]).
Keep it simple, if something needs doing, put it on a list, then do it. If your list is longer than five items, start a second list which you are not allowed to start until you’ve completed the first list.