What's the best way to make an attractive game cover in order to get people's attention?

Define "best".

Paying a professional would probably yield the best results. But since this is Itch you probably are asking for the easiest method of how someone not knowing how to do this could do this without paying moneys.

It is a combination of art and marketing and likely some mundane techniques that take the medium into account. A cover image has different size than a screenshot for example.

So I would start at looking at covers of popular games for your genre, if you find some common trends. Imitation should be a good starting point.

Also, you might want to attract people that would like your game and not attention for the sake of it. If you wanna do that, look at clickbait images. That is what they do.