Alrighty, so @pixelgeek is correct, we need to use a hide/show type of interaction to accomplish opening and closing a sidebar.
First
You need to add a div, give it a class of sidebar. Set it to position: fixed, and height: 100% - this will tell it to stick to the side of the page. You can use the controls in the position bar to set whether it stays on the left or right of the page.
Create an interaction on the sidebar called
hide sidebar on load, set its initial appearance to be horizontally -100%
Finally
Add an interaction on the element you want to open and close the sidebar when clicked
Tell it to affect a different element - the sidebar - and then on click move to origin
on second click - move -100% horizontally
Final result
Hope this helps you I have followed the same steps/principles from the example #4 on the interactions page and the video that @pixelgeek sent.
Building the header and footer fully before using symbols could prove to be the better course of action. For general structure guidelines Webflow also has awesome video tutorials here