To create a gallery, go to Add Media in your post editor; on the side you will an option called Create Gallery – all of your previously uploaded images will be available here, just select the ones you would like to include and click Create a New Gallery.

You will then be brought to the Gallery editor, where you can give your pictures captions or change the order of appearance by dragging them around. By default, galleries are set to Attachment Page, which allows users to flip through your images. You can also set this to Media File, which will display the image at full resolution, but if you only want the gallery viewable from the post, you can set it to None.
You can also specify how many columns you would like your images to be in. Checking Random Order will display your images in a random order each time the page is opened. 

When you are finished, click Insert Gallery; you will be able to see a preview of your gallery right in your editor. If you ever need to edit your gallery, simply click it and select the edit icon.