For starters, I put together this eportfolio rubric, lifting several things from various places on the web. Most blogs have students reflecting and posting artifacts. I wanted to have students go further by getting them to consider aspects of multimedia and reach. Although students are mostly doing text-based work now, they will soon be required to add aspects of multimedia, such as embedding a YouTube video and analyzing some aspects of it. One place that might happen is in our current "Bearing Witness" unit. Students may be able to post a video of a famous (or not-so-famous) speech and analyze it for elements of argument and rhetorical devices.
The analysis itself doesn't always need to be typed either; students may be able to start posting videos of themselves responding verbally to the prompt. One shortcoming in language arts and other classes is that we assess nearly everything through writing, even learning standards that have nothing to do with writing (such as literary analysis).
On the second page of the document below, students have their "tasks" for the first blog assignment. I am actually completing those tasks myself right now, setting up my blog and doing two blog posts. One of the posts requires students to embed a document and write a paragraph describing why you chose the piece, what skills it demonstrated, and what you learned in the process. In case you didn't notice, that's what I'm doing now!
I think this document has been a good starting point, but I think a more clear step-by-step handout walking students through the process (perhaps with shortened urls and/or QR codes linking to the appropriate resources or websites) would be a nice additional support. But then again, I don't want to be overloading them with handouts. Having this one handout with the links on the second side has worked fairly well considering the complexity of the task. And, as I discovered during my research for this unit, eportfolios are usually a messy process, especially when you are letting choose among multiple publishing/blogging platforms as we are.