One of the most important tasks to prepare for tea is to fill the natsume with the powdered tea. Depending on the shape of the container, the tea inside will have different shapes. When you are using a Rikyu style natsume the tea will be scooped into the container with the chashaku. A soft mound of tea like a hill is formed with the top of the hill coming approximately to the top of the open container and the bottom of the hill coming to the bottom of the line where the lid fits on. I have been taught various ways of doing this, and you will have to choose the one that works for you.
One way is to turn the natsume in your left hand as you scoop tea with the right, turning the natsume to get an even symmetrical hill. You can scoop the tea in without turning the natsume (my preferred method). With a full scoop of tea, gently lay it in the natsume and let it fall sideways off the tea scoop so that it stays soft and fluffy.
When you have a hira or flat natsume the hill of tea is shallower (see picture) and if you have a nakatsugi (middle cut) you make a sharp cone with the tea.
Some people then take a tissue or other tool and break up the clumps of tea on top, so that it is smooth. I don't like to do this, I just scoop the tea in and if I have done it right, there won't be clumps of tea on top. If you have dropped tea around the outside or on the lip that holds the lid, carefully wipe it off with a tissue. Try not to wipe the tissue inside the natsume.
It takes practice to fill the natsume correctly and beautifully. Remember, when you put the natsume out for haiken, the guests will be looking at how well you mounded the tea and how carefully you scooped tea and left the hill intact.