Hi, I'm interested in the following topic how to pay a salary to a partner. My partner is working as a subcontracted self employed for a flooring company but he's not running his own business. I'm from a different country from Europe. I would like to know the official way step by step how to pay me a salary to reduce his tax. I'd be earning around £40 thousands annual. What documentation's I need how to start this process? Thank you ever so much for any help.

Perhaps your partner needs to register his own business. If you want advice on these matters it is better to consult an accounting professional in the country you are operating in as countries have different rules

40 thousand? This sounds like a SCAM, big time. Possibly money laundering, but most likely a scheme to get money back from you, in smaller amounts, a variation on the African inheritance scam.
Your partner??? What do you mean partner? You worked together? Were lovers? Have you ever even met??

A partnership would be a schedule K-1 thus a Self Employed falls under a schedule C pertaining to Taxes in addition to this as an employee he would be considered a 1099 Misc he pays his own taxes, he cannot be a partner if he has a Tax ID as a Self Employed. Perhaps you should request addition information from him and then review the IRS website or an accountant in your state.