Two ways to achieve this are by using either a flash diffuser or a bounced flash.
bounced flash is diffuse but loses intensity
While it may at first sound counterintuitive, aiming your flash *away* from your subject can actually enhance their appearance. This causes the incident light from your flash to originate from a greater area, and is why portraits are usually taken with a flash that first bounces off a large umbrella.
However, bouncing a flash greatly reduces its intensity, so you will need to have a much stronger flash
in order to achieve the same exposure. Additionally, bouncing a flash is often unrealistic for outdoor photographs of people since they are no longer in a contained environment.