I want to learn to write Cantonese, I have learned Putonghua first.

Cantonese is only one of the Chinese dialects, its writting should be the same as mandarin. However, if you want to write it out in Cantonese, you need to speak Cantonese and the readers have to be Cantonese too. Believe me, it is very odd to the Chinese readers.

In oral English, "Don'cha wanna banna? The writting still be "Do you want a banna?". In the same token, Cantonese is a dialect, it is not used for writting.

Note: if you want to write Cantonese, you can use the Cantonese pinyin method, it is a software that you can configure to Cantonese pinyin.

你叫咩名
I'm not very familiar with Cantonese. The only knowledge I have about it were from Hongkong movies and TV show.
so as far as I know, the written Cantonese is actually from the prononciation of Catonese. It's kinda informal written style. so if you want to learn how to write it, maybe you have to refer to native Cantonese.
and also as far as I know, the offical written Cantonese is as same as the written Putonghua except it's in traditional Chinese. That may be proved by offical newspaper.
能看懂吗

i think you mentioned "乜"... it is a very useful word in Cantonese..