1). using the native WordPress gallery
– press the “Add Media” button above the text editor in the page edit mode.
– in the pop-up window, press “Select files” and choose the files you wish to add to the gallery (you can choose multiple files) and press “Open”.
– once all images are uploaded, press “Save all changes”.
– choose your desired gallery settings (make sure you choose the “Image File” option for “Link thumbnails to” if you want the images to display in an overlay when clicked) and press “Insert gallery”.
2). using the the built-in shortcode (this is the one used in the demo)
– perform steps 1-3 of the previous option, but after hitting “Save all changes” simply close the pop-up window.
– now open the shortcode generator (small button in the form of square parentheses right next to the image upload button).
– from the drop-down, choose the “custom_gallery” option.
– choose your desired gallery settings and hit “Insert”.
This option is slightly more flexible because it allows for better fine-tuning, e.g. choosing the exact image sizes, number of images to show, etc. Let me know if anything is unclear ;)
Should you have more questions, do not hesitate to contact me!