You do not use the word "alter" with tailor. When you want to get new trousers, from a tailor you should say:
عايزة افصَل القماشة دي بنطلون
you use "altet" only when you buy a trousers from "Wear shop not from a tailor" and you want to change it. In this case you say "عايز ابدل البنطلون ده" أو "عايزة أغير البنطلون ده"
Another sentence: وَسًعْهولي من هنا
You use this sentence when you try on البنطلون for the first time after he tails it in order to see if it fits you or not. This sentence means:
make it looser from here (you point to the place you want the tailor adjust it"
Another one: قَصًرهولي شوية
It means: make it shorter
The word قَصًرهولي is four parts. 1- قَصًر means "make it short"
2- the letter " هـ " means "it"
3- the letter "و" is sometimes written in Ammeyya instead of " the case ending الضمة"
4- the part "لي " means " for me"
So the sentence is: make it shorter
I hope it helps.