Think of a hardship or difficult situation you have been through in the past. Choose something that you have enough distance from to see it clearly and reflect upon it rather than something very fresh. Write down as many details from this experience as you can on a piece of paper.
Begin the sonnet with lines in iambic pentameter that define some aspect of this difficult situation. For example, if it was a death, describe the person who died, or your recollection of this person and/or the events surrounding the death. You may take up to the first quatrain or the octave of the sonnet for this initial description (the English sonnet has three quatrains and a couplet; the Italian has an octave and then a sestet).
Describe your actions or emotions in the present as they relate to this hardship in the following four lines of the next quatrain (for the English sonnet) or in the six lines of the sestet (for the Italian sonnet). Use iambic, five-stress lines as you have done before. For example, if you have described the death of a specific person in the past, explain how it affects you now, or how you have forgotten it, using specific details.
Define the situation even further in your closing line (Italian sonnet) or your closing couplet (English sonnet). The main turn in the Italian sonnet generally happens in the first line or so of the sestet, while in the English sonnet it usually occurs in the first line of the last couplet. You will have already performed much of the sonnet's turn by switching to the present in both cases, but you want to leave your reader with a lasting image or feeling in your final lines. For example, if you are writing about a death, switch back into past memory and write a description of your last glimpse of the person, or show your reader an object in your current life that serves as a constant trigger for this grief. Keep your lines in iambic pentameter.