It’s easy to start a new Build Diary thread, but you need to find the right bit of the forum. This depends on how you view the forum. I have my forum link set to open to the full forum display, rather than latest posts. You should get indications of unread posts from there, and it gives a good overview.
This link should take you to the My Build Diary forum. Just click on the ‘+ Post New Reply’ box at the top and there you go.
There’s a ‘Forum Actions’ section on one of the toolbars at the top of the page and you might find the ‘mark forums read’ option useful to start with unless you plan to read every single post.