You should be able to plant most plants in your cap layer. You don't need to push them down to the dirt, they will send roots there themselves if they are deep rooters. If you get any big swords or crypts you might have to get into the dirt layer in which you need to gently and slowly move the sand away with your hand, plant the crypt/sword, and push the sand back over. If you do get some dirt on the sand you can sprinkle just enough sand over it to cover it but don't add any more sand than absolutely needed. Don't start from scratch again. Just plant gently and you shouldn't make a big mess. You'll also learn dirt tanks IMO do better with a dark cap. You can see mine in the link in my signature.