First you need very sharp scizzors and long tweezers. I'm no expert but i've cut from most places on a plant with good success. I trim the tops of long plants, they get to big for the height of the tank so those are usually the first i cut, other trims i do are small branches or extensions coming off the original plant. Those branches usually don't look good so i trim them too. The reason i said sharp scissors is because the sharper they are the better the trim and it allows the plants to grow back faster, non-sharp scissors can lead to the top portion not growing anymore and the plant just branches out a new piece to grow. Also i said tweezers because planting with your fingers will lead to the plants being loose in the substrate and this leads to them breaking free and floating. So tweezers allow you the bury it deep and firmly so it won't become loose in the substrate.