Start with a complete request
我想買呢個 says “I want to buy this.” The phrase gives you a whole shopping request before you need to analyse each word. In a café, 唔該，我想要一杯凍奶茶 gives you a polite drink order you can hear from beginning to end.
A complete example helps you notice where the request begins and what changes between situations. Replacing every English word with a supposed Cantonese equivalent can produce an unnatural line.
Ask a yes-or-no question
有冇細碼？ asks whether a small size is available. 可唔可以試吓？ asks whether you can try the item. The questions do different jobs: one checks availability, while the other asks permission.
Play each as a full line, then choose the one that fits the situation. This teaches a useful question pattern without expecting you to construct a sentence from isolated vocabulary on your first try.
Add a polite opening and a repair line
唔該 can introduce a polite request or thank someone for small help. If the reply is too fast, 可唔可以講多次呀？ asks for another chance to hear it. A conversation includes both your planned question and the response you receive.
Practise a small loop: make one request, listen to a possible answer, and use a repair line if you miss it. The purpose is to become more comfortable continuing, one short turn at a time.