Instagram is a social network where you can easily share photos and videos from your smartphone. You can apply filters to your photos before uploading them. The Instagram app does not allow the creation of photo collages. The solution is to use another application to create collages on your smartphone and post them on Instagram. There are many applications for this purpose and some are free. Here's a non-exhaustive list:
InstaPic Frames allows you to edit photos with a number of filters and also create picture collages. You can choose from the several layouts and frameworks proposed by the application. InstaPic Frames allows you to create pictures in a square format (600x600px). You don't need to crop the image when you import it to Instagram.
Get InstaPic Frames for: