This is different from a paraphrase. In paraphrasing nothing in the original is omitted. But in substance writing you have to take only the essentials and express them in your own words. It is more or less precis-writing; though precis-writing is generally confined to prose passages.

The salient points of a poem are taken and put into a clear and coherent prose form. In order to do this you must read the poem again and again and understand it in a general way. After understanding its main trend, you put it as far as possible in your own words.

The resultant substance must be understandable without any reference to the original. In length it must always be a little shorter than the original, as we exclude in the substance much that is in the original.