In French, "vous" can be used instead of "toi" for this purpose. A seemingly rude example in English is when someone wants to instruct a lawyer, and writes 'I would like to instruct you.' I recognise the legal definition of 'to instruct', but this still sounds too forceful. What are pleasant, but formal alternatives?
I would like to instruct you is both polite and formal. Modern English has no alternate pronoun forms to denote politeness, formality or etiquette; it's correct to translate both tu and vous (and their nominative forms as well; you serves both functions) as you.
Instead, in English we can use different verb constructions to show politeness, similar to using vous + future tense for polite requests in French. This is not strictly analogous to the differences between tu and vous, but it's the closest thing we have.
And you've constructed your sentence in the polite register already. Would like is the more polite and formal version of want, and generally in English politeness and formality go hand in hand. Here are two translations for comparison. Apologies for any errors in the French; mine is extremely rusty. I also suspect my translation of instruct (meaning to issue a formal set of instructions) is quite poor, since I'd be much more likely to translate ordonner as order rather than instruct, but my French vocabulary isn't so hot. Please feel free to edit and change ordonner to the correct verb.
Je veux vous ordonner.
I would like to instruct you.
Je veux toi ordonner.
I want to instruct you.
Additionally, instruct is quite formal, but not forceful. Synonyms in this case include direct, order and command. The latter two are decidedly less polite than instruct, because these are the forceful verbs in English. Direct is just as formal as instruct and a touch more polite, but it's also slightly vaguer. If you say I would like to direct you, it might be taken to mean something like I want to guide you in a general direction as opposed to I want to give you instructions; an instruction is more specific than a direction.
If you provide a bit more context on exactly what type of instructions are being given to the attorney, I can suggest more nuanced synonyms. For example, brief may (or may not) be an appropriate choice here.