There’s no simple one-size-fits-all answer to this, but there are some best practices to follow. There have been several good posts about the subject, collected here:
Another good guide:
And an official guide from Glowforge:
Once your photo is edited properly, there are many settings options to explore. One interesting (though time-consuming) method involves using two passes:
As a last resort there are always people on services like Fiverr that can edit photos for a modest price.