I doubted because you posted anonymously, and instead of posting prepositional phrases, you posted complete sentences which use prepositions very well, generally.
However, your original question ("Could you please help how and where should we use the preposition while making the sentence?") seemed to show little understanding of sentence structure.
Yes, I have no confidence while writing a mail to my client or business. So, I would like to improve this skill with the help of you. The first problem which I felt was how to use preposition in the sentence...see in your previous mail, you have corrected 6 out of 10 sentences, which shows have only 40% of accuracy in using the prepositions. So,