Assuming you attached a script to the RigidBody2D for the bullet, what you do in the editor is select the RigidBody2D in the Node Tree.
Then, on the right-hand side (by default) are two tabs: one labeled "Inspector" -- how you set contacts reported -- and one labeled "Node".
Click the tab labeled "Node", and a list of different signals should show.
body_entered(body: Node) signal (it should be at the top), then double-click it. A dialog box will appear with the current scene's Node Tree, with the RigidBody2D selected -- it should say "
RigidBody2D (Connecting From)". If you have a script attached to the RigidBody2D, you should be able to click the "Connect" button at the bottom of the window. This will put the function
_on_RigidBody2D_body_entered(body: Node), or
_on_Bullet_body_entered(body) if the Rigid Body is named "Bullet", at the bottom of the node's script.