Drafts are saved onto your device. You won’t be able to access a saved draft from another device, even if you log in to your account. If you uninstall the app, your drafts will be lost.
Once you've
saved a post as a draft, you can share it the same way you would a new post. You can also discard posts you don't want to share.
To share a post you've saved as a draft:

Instagram app for iPhone
- Tap
at the top. - Below Drafts, tap the post you'd like to share.
- Tap Next, then add effects, filters, a caption or your location.
- When you're done, tap Share.
To discard a post you've saved as a draft:

Instagram app for iPhone
- Tap
at the top. - To the right of Drafts, tap Manage.
- Tap Edit in the top right.
- Select the drafts you'd like to discard, then tap Done at the bottom.
- Tap Discard Posts to confirm.