My best advice is practice! The process I follow is pretty simple once you get the hang of it. First I sketch something on a piece of paper, just the basic look of it. Figure out how many images and how much text you want, and how to organise them on the sheet.
Then I start googling images about what my idea represents. For example if your invention is an upgrade to an existing object, like a tool, you will type in the name of the tool and pick some images that would look good on your sheet.
For the text, you never want to have too much text because it makes it clustered and will make people not want to ready any of it. Like you said, keep it simple. Having to describe your idea on one single sheet of paper, including images and everything, really makes you stick to the most important stuff.
I hope this helps, good luck!