Do i make position one and position two then make a square and check any units that are in that square?
Get the mouse’s screen position (you can use UserInputService:GetMouseLocation()) when you start holding the mouse and and when you release the button. You can get the screen space coordinates of any instance using the Camera:WorldToScreenPoint() method (Camera | Documentation - Roblox Creator Hub). For every point, check if it’s in the bounds between the start and end mouse positions. If it is within the selection bounds, do whatever you want with it!