So ...

Upon activation, you need to go to Post Grid => New Post Grid to create a new grid layout.

First, you'll need a name:

Next, select the "Query Post" to define how do you want the plugin to retrieve the posts for display.By default the plugin will display your posts, but you can also select pages or custom post types:

And you'll be able to define other additional options like post status, number of posts per page, query order, excluded posts etc.

Next, go to the "Layout" tab to select how you would want to display your grid. Define your content layout (flat, flat center, etc) and choose a skin for your template. Just hover your mouse over a skin and you'll see how it works ...

Additionally, you can also click the edit button which will open a layout editor screen where you can create your own layouts if you want.

Next, go to the "Layout Settings" tab. You'll see a lot of different options and you can freely play around with these settings to find the best fit for your needs. The most important option here is the "Grid Items Width" setting which will define the total width of your grid on desktop, tablets and mobile. The default settings should work fine for most of sites, but if your grid layout doesn't fit on your page, this is where you can fix that:

Finally, Select the "Navigation" tab to select your pagination style and once you are done, click the "Publish" button.

OK. Now let's see the magic ...