A metaphor is a comparison that does not use the words like or as. A simile is a comparison that does use the words like or as.
To change metaphors into similes, we first have to understand the meaning of the line in question and locate the metaphor. The first example is as follows:
As the stir cracks and crazes their enamel.
The "stir" in this line means breeze. The speaker is saying that the breeze cracks and disorders the branches' enamel. Enamel in this line is a metaphor for the ice coating the branches. To turn this line into a simile, you could say that the "stir cracks and crazes" the branches' icy coating, which is brittle like enamel.
The second statement says,
Soon the sun's warmth makes them shed crystal shells.
In this case, the speaker means that the sun's warmth makes the branches shed their icy coats, which he refers to as crystal shells. You could say, "Soon the sun's warmth makes the branches shed their icy coatings, which fall to the ground as if they are crystal shells.
The final statement comments,
Such heaps of broken glass to sweep away.
This means that the icy covering that cracks and falls from the branches to the ground reminds the speaker of broken glass that needs to be swept away. To turn the statement into a simile, you could state that the ice on the ground that once encased the branches looks like broken glass that needs to be swept away. In all three cases, you are adding a like or an as to the comparison.