It's not any harder than making a batch of cookies.
But, you do need to follow the directions. When you put your dirt in, did you mash it down to compact it? Did you use a 1" sand cap?
After I do that I slowly add a little water until the dirt starts to get wet. To see this better, in one corner, scoop the sand off the dirt. I'd tell you to to get just enough water that the sand is only damp.
Now, just move a little sand were you want you plants and add them. Then fill in around the plants with sand. Once your planted put in just enough water to cover the full height of the plants. Now, if you planted anything too deep(buried the crown) gently pull that plant up a bit.
At this point I won't fill the tank yet. I'll keep the water level low and let the plants get a lot more light by being in shallow water for a few days. Also, If I made some errors in planting I'll just take the water level back down and fix.
Remember plants will grow an spread, leave them space.
I think the biggest mistake is going too fast.