The first step you need to is to register as a ACSP.
Information can be found here on how to do so https://www.gov.uk/guidance/applying-to-register-as-a-companies-house-authorised-agent
Once registered, you’ll be provided with a new digital account and unique identity number
Then follow these steps.
1. Tell Companies House you have verified someone's identity
2. 
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5.
6. You confirm various pieces of information about the client, then
7. Then it asks you information about the document you have checked.
8. If successful you will be shown this picture below and the client should be sent an IDV to their email address.
More information can be found here:
https://www.gov.uk/guidance/tell-companies-house-you-have-verified-someones-identity