How do I decorate the windhelm house?

Solution 1:

According to the Elder Scrolls Wiki:

Talking to the Jarl's Steward, Jorleif, enables the purchase of decorations for Hjerim; however, the quest "Blood on the Ice" must be completed before buying decorations and the cleanup package from Jorleif, or else glitches ensue.

In other words, you need to finish the murder investigation before you can purchase decorations.