It's quite simple. Just open the app where you want to post, look for the upload or post option, and select the collage from your gallery.
To post a collage on story, first make sure the collage is saved on your device. Then, in the relevant app, click on the option to add a new story. From there, you should be able to browse and select your collage. Some apps might also give you options to add text or filters before posting.
Well, the process can vary depending on the platform. But usually, you need to find the 'create post' or 'share' button. Then, it'll prompt you to select the collage file from your device's storage.