Using a floor planning app like RoomSketcher makes it easy to design a site plan. Start by drawing the outer walls of the buildings you want to include on the site. Use measurement tools to make sure you get the buildings the right distance apart from each other.
Then draw up the outside perimeter of the plot. Work your way out from the buildings and add in driveways, terraces, etc. If you are using the RoomSketcher App, you will find plenty of tips and tricks on how to create your 2D site plan in this help article.