how to make an easy dc/dc transformer to run a lenovo laptop on 20 volt from a 12 volt battery
I am a bit confused here. You are saying that you'd like to learn and at the same time posting a link to cheap ready-made DC-DC converter.

If you simply need to power your laptop then there are tons of suitable DC-DC available for under 10 bucks. Just make sure it accepts down to 9V input (the one you linked doesn't) if you planning to use it in a car.

If you do want to make your own then be warned - buying components would probably cost several times more.

Having said that, there are many DC-DC regulators that require just a few external components. For example LM2587/LM2588 from Texas Instruments. It's  datasheet is full of useful examples that should be easy for you to replicate. Most important, the datasheet has the list of suitable standard transformers/inductors, which is the trickiest part to make yourself.