This is the fundamental Makko no Suburi.

Well then, please allow me to explain to you Tenshinryuu’s training method of 素振り (suburi – “practice-swinging”).
This is going to be about 真向の素振り (makko no suburi – roughly “frontal suburi”).
First, assume Seigan. Then from this stance, place your right heel onto your left ankle to assume the stance of 撞木足 (shumokuashi – “bell hammer feet”) and at the same time hold up your sword in 上段 (joudan).
This stance in which you hold up your sword like this is called 満月の位 (mangetsu no kurai – “full moon stance”) in Tenshinryuu. Make sure the tsukagashira goes right above your forehead, and don’t lower the kissaki.
From the opponent’s perspective, they should be able to see only the tsukagashira, but since this depends on their height there is no specifically fixed angle for this. It’s fine as long as you keep in mind not to lower the kissaki too much.
From here stretch both of your elbows, take a step with your right foot and slash downward.
As the target for the slash/cut, you should aim at roughly the hips’ height. There is no need to go all the way down with the kissaki. It should be fine if the kissaki stops at around the height where it would barely or almost hit the opponent’s wakizashi.
When slashing/cutting down, please be sure to bring your left fist even past your right knee and essentially move your balance to the front so you can reach as far as possible with your slash/cut.

Pull back and cut down.

Important points

We would like you to pay special attention when you’re holding up the sword.
If you swing it back forcefully, it might work just fine with something as light as a wooden sword, however, when swinging a sword in actual combat you might end up delaying things.
So the answer to how to properly swing back the sword lies in pushing in with your left hand.
If you push the tsukagashira with your left hand, the kissaki will swing backwards. With this posture, if you now slightly raise your right hand, the sword will go up naturally without using any force from both of your hands.
After that, stretch both elbows as you swing down the sword to complete the cut. This is what it should look like.

Good and bad examples

I’m now going to use a slightly heave metal fake sword.
First, I’ll show you some bad examples.

(I wonder if you can tell the difference…) All right, so next, I’ll show you some good examples.

Could you tell that the speed of the swing back was different?

(Do you know what I mean?) Please watch from the side.

This is when I use force to swing back.

(This is… ah, you already know?) One more time.

I just swung a bit quicker to make it easier to see, but this is when I don’t use any force from my arms and instead swing back utilizing the principle of leverage.

If I swing back forcefully and quickly…

With this kind of method I will end up tiring myself out and get slower.
In contrast to that, when using the principle of leverage I slash/cut down much more smoothly.

I swung quicker to make it easier to tell, but I think the difference should be pretty clear.
Swinging back by using the principle of leverage like that is the secret of 素振り (suburi) training to be able to use even a heavy sword in a smooth manner.
素振り (suburi) isn’t specifically about swinging this or that many times. Instead, we would like you to do reps according to your stamina.
What is important at first is assuming the right postures and swinging down with 剣気体一致 (kenkitai icchi) or 剣気体一丸 (kenkitai ichigan).
Please try it while keeping in mind to cut down accordingly to the physical work using as little force as possible.

About the feet’s distance

Assume 撞木足 (shumokuashi) and cut down.
Regarding the distance of your feet at this point, as a principle, if you kneel down with your hind leg, you should be able to fit one fist between the heel of your front foot and the knee of your hind leg.
There are two types of training, so first is the 素振り (suburi) on the spot. In this case, it’s just the one-fist rule.
The other one when you take a big step with your right foot and slide forward. Since, when you take a big step with your right foot, it would naturally leave a big gap between your feet, you have to bring your left foot to the front as well. This is the more difficult 素振り (suburi).
In any case, at first please conduct your training while paying thorough attention to your feet’s distance and checking whether your feet are positioned correctly when you move right after cutting down.