I'm new to DW, only 120 hours under my belt, but these things I've come across might be useful to you:
1. If you let the AI make some fleets it wants by giving it some ships at setting them to auto you need to follow what it expects from escort, frigate, destroyer etc designs or it won't know what to do with them. If you don't, and I now don't, you can design your own ships for specific roles such as a tank, a missile boat, a boarder and so on and forget about the predefined ship roles the game has.
2. A lot of techs, most of them in fact, have a series of improvements following on from initial unlock. You ships and bases get these upgrades for free if they have the base component installed, so a reasonable approach is to grab a series of new unlock techs (a particular shield, torpedo and engine type for example) then upgrade your designs with these and retrofit. This way you can research a large list of improvements that further enhance your fleets without needing to retrofit further.
3. An easy way to build a new design is to use the copy button on an existing one and edit it, although you've probably noticed that from Larry's videos.
4. Another really good tip from Larry i find is to use the Auto upgrade selected designs button then hit edit (which you can do 'cos no ships of that design have been built yet) to teak to taste. The usefulness of this as a beginner i find is that certain things that you've forgotten you've researched or don't understand the significance of yet get automatically added or upgraded in the design for you which you might otherwise easily miss. Good for learning about things.