It's simple. Visit www.kiwiforgmail.com and download the app. There is a version for Mac, as well as for Windows. You can get a free 30-day trial, afterwards it's $9,99 per year. However, there is a Lite version, which is completely free, but it offers less features. Here you can see the differences between the Lite and the Premium Version.
I've always used an external email app - Eudora, Lotus Notes, Outlook, etc. When faced with using Gmail, the idea of having to keep remembering to have a browser window open all day was quite frustrating. I searched for some options and came across Kiwi. Kiwi was simple, and provided enough features to give me the standalone feel, but the power of Gmail. I really like that I can manage multiple gmail accounts in one program. Better than having multiple browser tabs open all day!