It is perfectly acceptable to request feedback from those who you have had transactions with. Even friendly reminders are often appreciated as life gets busy.
If the other member is unsure of how/where to leave feedback, you can direct them to the other feedback FAQ "How do I leave feedback?".
How do I leave feedback?
In the navigation just below the site name and login, is the link "iTrader". It has a small arrow with drop down options such as searching for a members feedback profile, etc.
You can also leave feedback by going through a members profile. On the left side of the the profile, in the box titled "Forum Info" you will find the link "Trader Ratings". Clicking on that will bring you to the feedback profile where you can view and submit feedback.
Feedback needs to be factual and without emotion. Please remember that name-calling is against the rules anywhere on DS, including feedback.