The video below is for amending an existing SH01 with an error, but all steps are also applicable for filing a new SH01 with Companies House.
If you're filing a new SH01, please ensure to choose the 'Share Capital - Return of Allotment of Shares' link, instead of amending an existing one. All follow-on steps are the same.
If you're uploading an SH01, you should also accompany it with a shareholders resolution. These resolutions are uploaded in a separate part of the Companies House website.
Here's a step-by-step guide for how to upload Shareholders Resolutions, SH02s and other documents that can't be uploaded on the regular Companies House site.