There are plenty of DIY shed tutorials online, but what's often lacking is how to properly construct the foundation to ensure it's square, level and less prone to rot. This videos shows you how to accomplish all three.
Take your time and don't rush the construction of your shed. The easy way to check that your base frame is square is to measure corner to corner and adjust until the two measurements match. Use 12mm plywood to temporarily lock in each corner after it's square and use 100mm deck screws with washers to attach the corner 50mm x 150mm to each other.
Raising the platform off the ground will protect it from rotting, but it can be difficult to make it level. Use a bubble level and clamps to adjust the height. For a final check, place the level across the corner so it rests on opposing sides and then clamp it tight and attach your frame to the footing.
The best tip here is how to set the floor joists when you are by yourself. Use a clamp to attach a 50mm x 100m piece of timber to the underside of the frame and use that to rest your floor joist on as you position it into place and secure it.
This video has a lot of other tips including best practices for measuring joists and how to prevent critters from making a home underneath your shed or tiny house.
How to build a level shed platform [YouTube]