this is your access code and instruction on how to do use it. 請問 how to do uset it 為什麼放在on後面?

In this pattern the object of the verb is an interrogative pronoun or adverb, followed by a to-infinitive:-
eg:- 
Do you know the access code's instruction on
(how to do it) ?
eg:-
You must learn the access code's instruction on 
(how to do it; not how to do "use" it)
eg:-
interrogative pronoun or adverb =in what way or manner; by what means=Tell me the instruction "on" how to do it. placed after "on"
=how to do it.
=Tell me how you can do it.
=The access code instruction told me "on" how to do it, not how to do "use" it.

🔷 instruction(名詞) {on(介係詞：<釋義> 關於，有關) how(副詞) to do it} (介係詞片語)，這介係詞片語修飾該名詞。
《美辭典》in‧struc‧tion：COLLOCATIONS-1. COMMON ERRORS
► Don’t say ‘instructions how to do something’ or ‘instructions to do something’. Say instructions on how to do something.
# COLLOCATIONS-2. gives instruction：• In Delhi, an International Labor Organization-backed pilot program gives instruction on how to rehabilitate discarded shoes.
《日辭典》instruction：釋義-1. 〔～s〕指示書，（…についての/…のための）説明書≪on/for≫
例. instructions on how to care for animals 動物の世話の仕方の説明書

不在後面？放在那裏？？？？？？？？？？？？