This post isn't very good, and I'm not proud of it.
The first step to making a mockup is to open up the inpect menu, you may be able to do this using ctrl+shifT+i(or f12)
this will be different from browser to browser.
once you see inspect element open, simply go to the page you're making a mockup for(ex. MyStuff; or if you're making a new page, go to the page that looks the closest.), and press ctrl+shifT+c, and click on the area that you want to change(ex. the + New Project button.
now that you've selected the element, let's learn how to edit it! You should see the element that you clicked on(using the ctrl+shifT+c command) highlighted in blue. Inside of the element(if you selected text, or even an image), you should see a darker area, that isn't red, double click on it, it should now be hilighted, now type in whatever you want(if it's an image, make sure to put in an image link, ex. https://imagehoster.com/png/png.png; if you clicked on a link(in the "" in "a href=''", unselect it and select the part ""here"").
anyways... if you don't have the monna lisa yet, just give up /s
