Hold onto the ball carrier to buy time for support
The first supporting player must assess the position of the player on the ground and decide how to act. #1, they can choose to "take the space" above the ball. This act is designed to secure possession, with the first supporting player keeping their shoulders low, chest unexposed (opening up the chest to the opposition allows them to get underneath) and head up is important.
This player should not allow their shoulders to fall below their hips, as this will signal to the referee that they are attempting to prevent a contest by going off their feet at the breakdown, an automatic penalty. Instead, the first supporting player should "clamp" onto the player on the ground to create stability, leaning slightly forward with the ball in line with the hips similar to a scrummaging position.
A clamp is exactly what it sounds like, a low position over the ball with a wider stance and head up. If possible, grab onto the ball carrier on the ground. This isn't technically allowed under the Laws. But it's very seldom called by referees and is the best way to guard against a counter-ruck.