1. First locate the Grid Gallery widget under the "Galleries" section of the widgets menu while in Edit mode in the Pages Tab. Drag the Grid Gallery widget onto the page where you would like it.
2. Click on the gallery widget and in the content menu to the right of the screen for that widget, click the large orange "Edit" button.
3. Once you click to edit a new window will pop up. Click the button that says "Add Images" at the top left of that window to pull up the Media Library.
4. Select the images you want to add to the gallery and hit the green "Apply" button at the bottom of that window.
5. Arrange the images in the gallery however you would like by dragging them, and then click the green "Apply" button.
6. After you apply the images, the gallery will automatically format. Using the tools to the right of the screen in the content menu you can edit the look of the slider. You can adjust the height of the rows, the margins between the images, and even format the layout of the bottom row of the gallery. You can also click the design option at the top of the content menu to change the design preset.
7. Once you are happy with your gallery, be sure to save your changes!