Now that the feature is here, all you need to do is tap the photo icon in the tweet composer and add the media of your choice. The content you add will show up side by side or in a grid-like format, depending on how many images, videos, or GIFs you add. You can see how this might look in these tweets shared by Twitter.
When I tried out the feature for myself, I found that you can add up to four pieces of content in total. It’s unclear whether tweets shared off-platform will support this feature, though. I included Twitter’s multimedia tweet in this post, but it only shows one of the two pieces of content included in the tweet and appeared the same way when I shared it with my colleagues on Slack.