Step 1: Find the things you need
Step 2: Prepare the boxes
If you use a milk-carton, like I did, you need to cut them down the middle so you have two "boxes". You can also use a carton that is already open.
You then need to cut some holes in the bottom of the boxes with the sharp knife to let out access water so the seeds won't drown.
Step 3: Sow the seeds
The most frequent process is starting with a thin layer of dirt, after which you carefully sow some seeds. Then cover the seeds with a thicker layer of dirt, gently push the dirt down and give the seeds some water.
Water the seeds frequently (see instructions on the seed packet to see how many times is recommended) and keep them out of harsh sunlight.
Step 4: Take care of the seeds
Step 5: Re-plant
You can plant the small plants in any shape you like, like this X shape or any other shape you like.