1) To include multiple images or videos in a single Instagram post, tap the + sign.
2) On the next screen you’ll see an icon of stacked squares that represents a photo carousel post. Tap on the icon
3) Scroll through your image library and tap to select up to 10 images and/or videos. When you select an image, the number in the top-right corner of the image will tell you the image’s number in the slideshow order. Once you’re happy with the images and videos you’ve chosen, tap Next.
4) On the next screen, you can edit your images and apply filters. When you first navigate to this screen, any filters you choose will automatically be applied to all of the images in the post.
5) If you tap on an individual image, you can edit it and apply filters individually. When your editing is complete, tap Done.
6) To change the order of the images in your slideshow, tap and hold an image you want to reorder. Once you see a trash can icon at the top of the screen, drag the image to the left or right to reorder your slideshow.
7) On the next screen, enter your caption as you normally would. If you tap on the image thumbnail while writing the caption, you can scroll through the slideshow and tag users in individual images.
8) The tap Share and you’re good to go.
SOURCE: Instagram Blog