There was a loss of separation event when a controller was concentrating on upcoming crossing restrictions with several DTW arrivals. The conflict was with enroute traffic.

Narrative

I was working Appleton (87) Sector during a slow traffic volume time and I made a mistake that resulted in loss of separation. The incident happened as followed. We were metering to DTW and I had three DTW arrivals stacked vertically on top of one another and I needed to have all of them AOB (at or below) FL300 before making hand off to ZOB ARTCC. I cleared Air Carrier X direct CVG VOR at FL320 who was moving from east to west at the time. Initially; I descended Air Carrier Y to FL330 to stay on top of Air Carrier X. For some reason all I could think about was I needed to get my DTW arrivals stepped down to meet the LOA (Letter of Agreement) of AOB FL300. I descended Air Carrier Y to FL290 northbound direct BOBCT Intersection. I then descended another DTW arrival to FL330 when I noticed Air Carrier Y left FL330 for FL290 overlooking Air Carrier X at FL320 westbound crossing in front of Air Carrier Y. A few moments later when I realized what I did... I tried to climb Air Carrier Y back to FL330 hoping I could get the separation I needed. Air Carrier Y said he was climbing to FL330 on a RA he got from TCAS. Air Carrier X then said he was descending to FL310 from a RA he received. Loss of separation occurred of 4.375 miles and resulted in a pilot error. I don't know why I overlooked Air Carrier X when I was staying above him initially. All I can think is that I was too worried about getting all DTW aircraft stacked on one another below FL300. For now on I will use a J-Ball to keep me focused on the conflict I'm trying to separate from. I will definitely change my techniques after this pilot error. Using a J- Ball on Air Carrier X would have helped me tremendously. I would have realized the aircraft that I needed to be cleared of to descend all of my DTW arrivals down. I wouldn't have overlooked it after initially stopping an aircraft 1000 feet above from the start.