Do I just describe myself, or do they want certification that I attended some course like anger management (I haven't)?
The best way to fill out these forms is to list achievements rather than skills. Focusing on interpersonal skills, have you done any training or presenting, say internally to the team or at conferences? Have you done a public speaking course or communication course? These are all great because they show you went above and beyond the expected level and attained a goal.
Failing that, focus on things that set you apart from others. For example, if you were in a software development role but negotiated the purchase of expensive software, list that. Did you handle customer support directly? Did you liaise with other groups in the organization, e.g. security, localization or QA? Did you talk to other development organizations in other departments, offices or countries?
I assume you have not been in a role that required significant interpersonal skills, e.g. management, sales or support. If so, focus on the skills that made you successful in these roles. For example, if you were a manager, how did you motivate employees? Did you interview any new employees? If so, what did you look for. How did you sell any ideas to senior management? How did you communicate bad news to the team?
Remember to include things outside work, too. Do you have a blog or Wiki? Are you involved in any open source projects? Do you attend user groups? While these are not considered interpersonal skills, these are areas you can look for achievements or things that set you apart.