Building a foundation for a cabin is essential for the life of the cabin. There is a reason why "building a good foundation" is an effective metaphor--its true. I offer my method and an alternative here so you can pick which fits you best. I picked the expensive, more durable way but the other way is good as well.
Things You Will Need
Chainsaw or axe
Heavy equipment or a shovel
A few clear days (weather and time-wise).
Step 1Clear the site. Taking a chainsaw or an axe clear out all heavy brush and trees from the area of the building. You don't need to remove vegetation, you'll do that as you dig.
Step 2Stake off. Hammer wooden stakes into the ground for the four corners. Make sure the wall lengths are close.
Step 3Square. Determine the squared length from diagonal corners. The result will be the square root of the sum of the side squared and the adjacent side squared. In other words if you long south wall is A and your short west wall is B, the squared distance is the square root of A2 + B2. Measure from one corner to the opposite diagonal one. The distance should be this number. If not, move the stake and remeasure both the diagonal and the sides that the corner affected. Repeat for the other diagonal.
Step 4String. Run your twine around all stakes to create a rectangle. Remeasure again for good measure.
Step 5Spray paint along the string onto the ground. Remove stakes and string.
Step 6Dig. Using a shovel (god I hope not!) or a backhoe, dig the square trench with lines about 2 feet wide. The spray painted portion should be dead center. Dig it to about two feet deep or more depending on your frost line and location.
Step 7Hammer corner stakes to the depth you want the concrete. Measure from the trench bottom to the top of the stake. Do this for all four corners. Run a line and place a line level on it. Hammer the corners until you get a general levelness. You should really do this with a laser level but they are expensive. You really also need a line level on all four lines but you can make up for out of level to some degree later.
Step 8Hammer more stakes between the corner ones and make sure they are level to the line. Once you have a stake about every 2 feet, remove the lines and levels.
Pour the concrete. I hope you have the good fortune to have a truck haul it in. If not you will need to mix it wheelbarrow by wheelbarrow.
Once the concrete is high enough you will need to use a trowel and level it up to the top of the stakes. Smooth it as much as possible.
Let it dry for several days before moving to the next step.
Step 6 - Dig round holes about 2-3 feet deep or more and purchase and insert sonotubes which are concrete form tubes made of cardboard. Dig these at set intervals. About 4 feet apart or so (whatever works for you).
Step 7 - Level the tops of each with strings and line levels. You will want the concrete piers to extend a foot or so above ground level. These will serve as your foundation and foundation walls.
Follow the other steps as normal except between steps 10 and 11 you will want to bury some i-bolts to place your wood on top.
A foundation can be as complicated or simple as you want it to be. In older days people made foundations by digging holes and filling them with large rocks. Luckily we have a little more technology at hand.