This long activity belongs to the “deciding on actions” stage of sprint retrospectives. It works well with teams that are forming, storming, norming, performing, or adjourning.
Write the issue(s) the team wants to tackle on the whiteboard. Have them suggest steps to solve the problem. For each suggestion, ask if it can be completed in one iteration. If not, cut the action into smaller steps. Repeat this until you have several realistic options. Have the participants dot-vote to pick two to three for the next sprint.