How should I begin to approach a practical criticism?

First, you should always begin with a brief summary of what the text is about. You need to show that you have understood the text. As you do this, think about what the text is doing; is it satirical, portraying a power struggle between two people etc.? This is how you write your introduction. Then, for the rest of the essay, you demonstrate how this is shown in the text with detailed ananlysis and precise examples. You should think about language (words chosen), form (what the text is) and structure (where things happen). There is no need to write a conclusion. 

Whilst writing, you should aim to use technical language and terminology; the more precise you are the more you will be able to say. 

The most important thing to remember is that it isn't enough just to spot the features of the text; you have to comment on the effect that each one has and why the writer has chosen to use it i.e. why it is there. 

Take time over it, go through the poem carefully, look for clear examples and go from there. Ask yourself questions as you are going through, such as:

Do the words have any particular connotations?

What is the relative length of each line?

What tenses has the writer used?

Is there any figurative language?

These can be applied to any practical criticism, whether it is prose, poetry or drama, and will make sure that you think about structure, language and form when writing your answer.