How do I go about asking people for Google reviews?

Ask.  That’s the first step.  If you don’t ask, you won’t get the reviews.  However, many people aren’t familiar with how to even give a Google review because they have not given one themselves.  If you’re on of those people, it’s a good idea to get online and give a few reviews yourself so you see how easy it is.  Once you’ve done it a few times, it won’t feel like you’re asking for such a monumental favor.  It just takes a second yet it’s worth so much to your rankings on Google, particularly in the map.  Try it.  It’s not as difficult as you might think.