I am currently sponsored on Instagram by Sport Horse Essentials and a horse treat small business. I started by making my account on Instagram pretty big and posting a lot. Most companies will have a form that you will fill out to apply. I am almost about to sign with another company as well that is based out of Canada. Good luck
You can start an instagram account to share your results, take great pictures... Post good stuff about specific products that you would like to get sponsored by.
Don't hesitate to contact companies you would like to be sponsored by, send them nice messages to present you and what you can offer to them 🙂