First, and perhaps most important, the person has to be interested in receiving assistance. So get permission. Help without permission is a violation of your partner. Permission can only be granted if your partner has the option to refuse the help. On occasion a partner may want to struggle a little longer or get their ideas or skills more organized before they are ready for further input.
Mutual respect includes the ability to wait.
Assuming you have permission to provide assistance the next step is to make the help constructive. This is often more an issue of style than content. The best ideas presented poorly get lost. The input needs to be realistic. When Chelona began helping me write there was no way she could turn me into a Hemingway.