To create or edit a seating chart, start by going to your Venues tab. Then, click the "Seating Charts" link on your venue. Seating charts come in two flavours: without sections or with sections. When you start a new floor plan, you get presented with this screen to make a choice:
If you choose the first option, you'll immediately get all the tools to draw bookable objects: seats in rows or as a block of rows, seats at tables, booths and general admission (GA) areas:
Pro tip: Some tools have a little > arrow next to them, indicating there are multiple variants of the tool available. For example: the GA tool allows you to draw rectangular, round or polygon GAs.
On charts with sections, things are a little different. There you need to first draw a section, and then fill it up with bookable objects.
First step: Use the section tool to draw a section (either as a polygon or a rectangle):
Second step: go to section mode, either by double-clicking the section you just drew, or by selecting it and using the Edit Contents button on the top right:
Third step: In section mode, you see a large version of your section. You now have all the tools you require to draw bookable objects within this section:
Final step: When you're done, click the "Exit Section" button (top left) to go back to your sections overview.