Creating OER can be done in several ways and may be easier than you think.
If you are an individual looking into create OERs, think in simple terms. Create an OER by digitizing your existing materials and putting an open license on them.
Of course, you can also create a resource or tool from scratch and add a creative commons license giving others permission to revise and/or remix the content into something different.
Or take an existing OER resource and revise and/or remix into an improved version. Then share it as an OER under a similar or different version, based on a creative commons license.