How to make a conserved element (menu bar)

How do I make an element that is conserved between pages? e.g. a left-hand menu - so that I don"t need to manually update it on each page and so that if I update it once then it is updated on all pages.

I know about the slidable plugin (I’m presuming this does this somehow), but I’m hoping to do without plugins for now (unless unavoidable).

I presume that once I had some a conserved menu then I could make it conditional so that the current selection is highlighted, or not available, when you are on that page.

For that you use Reusable Elements. You’ll need to add the Element to each page, but the content of it will be updated everywhere.