To upload a match/demo: (remember to pick either your personal or team/org, where you want the demo uploaded)
1: Click on "Matches" in the left menu.
2: Click on "Add match".
3a: To upload a demo that you have locally on your computer, click the area saying "Drag files or click here", then select a demo file and press the button "Submit file(s)".
3b: To upload a demo with a direct link, click on the tab saying "Direct link", enter the URL in the text field and click the button "Submit".
3c: To upload via a CSGO share code (such as CSGO-Ab1cD-xYz23-7bcD9-uVZ23-12aBc), click on the tab saying "Sharecode", enter the sharecode in the text field and click the button "Submit".