It has been made very obvious that Mahirji hasn't had any sort of romantic entanglements in the past, we have all realised this very early in the series.
On the wedding night Mahirji brought the kada for bela and proceeded to make her wear it. But he wasn't able to figure out the simple lock on a basic kada. This clearly showed that he doesn't have much experience with these things, acceptable.
How come suddenly he is not only able to drape a saree perfectly on her but also knows where the pallu pin is and what all other things she needs along with saree? When he goes to bring her a change of clothes he says sirf saree se thodi kaam chalega! And he brings her a well coordinated saree accessories! And those things weren't in one hanger, they were placed separately! And all this he brings in a jiffy because bela isn't feeling good, so he doesn't think about it much, he does it as a reflex!
But he couldn't tie a simple dori on Anu's blouse, even in disguise as a woman, even to save the situation.
Now he is trying to fix a waist belt on bela like some kind of expert!
The question is where did he learn all these technicalities regarding feminine clothings and jewellery?
It's clear he didn't know all these 4 months back. And Mahirji and bela obviously change in different rooms though Mahirji is known to change in front of her, it's never been the other way around, there is very less likely possibility of him seeing bela wearing these articles for him to understand the process. Let's be honest we women have some what complicated jewellery and clothing system, at least it must seem that way to the opposite gender.
So? How?
The only logical explanation is due to his excellent observing skill. He may not have witnessed her wearing these but he definitely has observed her very keenly!
Bela ki pallu ki pin kahan lagi hain, saree ke saath aur kya accessories lagengi, and what colour saree-blouse combination she prefers,kaunsi jewellery kahan pehnani hain.
This can't happen over night, we girls know this is a art we can master only after repeatedly doing it, so for him to do these things with ease means Mahirji ne bohat zyada hi notice kiya hai biwi ko, aur bohat pehlese hi observation chal rahi hai!
So, The question isn't where did he learn all these technicalities regarding feminine clothings and jewellery?
The fact is he has only learnt about technicalities of BELA's clothings and jewellery!