To practice using "should", make a list of sentences which refer to specific circumstances on the board.
I have a headache.
We don't have any bread.
I want to improve my English.
The students have to use a sentence with "should" in response to each statement. You could, for example, write each statement on a card and hand them out. Divide the students into pairs or groups. The students take turns choosing cards and giving advice using "should".
You could try something similar for othe modals.
Here's a way to practice, or teach, using "can" for ability. Write these words and phrases on the board. You can write or type them on cards as well. Hand out the cards and then the students take turns asking each other questions: Can you ride a bike?
ride a bike
play the piano
build a website
I find at lower levels it's really about being creative. Language is a way of demonstrating the basic things we all have in common. At more advanced levels, planning lessons and deciding how to teach is much more situational: What's the situation? Who are the students? What is their purpose in learning English? What is their background?
Post any follow-up questions you have. I might have another suggestion or two by reading a follow-up question.