There are many ways and many reasons for digging a hole, but the tools are the same: spade, shovel, and maybe a pickaxe. However if you need a pickaxe, it is better to hire machinery to do the job. You can save a lot of time, physical efforts and even save yourself from pain (backache).
So, this instructable is about to do it by hands and with a lot of physical effort. What is - anyhow - a great workout
Step 1: First Step: to Measure
Use your tape measure to sign the four corners of hole. You can make marks with your shovel, or just put four stones there for each corners. If you need/like to be more precise, then use poles and lines to mark sides and corners.
Do not forget to use you measuring tools (tape measure, spirit level meter, else) during the digging process. You can save lot of your efforts with just this little care.
Once you dig the hole, you will have to shovel away the dirt that was scooped up by the hole.
You might be surprised by the fact that the volume of loosened earth is greater than you think. And the deeper you dig, the more storage space you need, and you need it close to the hole. If there is no such area then you will need a wheelbarrow too.
So you have your determination, your hands, tools, the measured place of hole, and storage space very close to it. You are ready to start digging now.
Step 2: Start to Dig
You will see how thick this layer is from its color, but - as a common rule - I suggest to put aside a spade deep layer.
I mean as thick layer as the length of spade head is.
Step 3: Now the Hard Work Starts
The loosened dirt may contain big dirt-nuggets - as I you can see on this picture - if the soil is dry.
Cut them into small pieces with your spade. It makes the shoveling easier.
Step 4: Deeper
Now you can start to loosen a new line. Then cut it into small parts and throw the dirt away again.
Repeat it until you have a clean and nicely leveled floor in the hole.
Then you can start a new layer deeper.
Do it over again, until you reach the wished depth of your hole.
Step 5: Just for Fun
I finally finished the hole in hot weather of 40C°/ 104F°. So I let the body hit the floor.
It was cool.