Carefully turn the MacBook over, so that the lower case lays flat.

Raise the upper case/display assembly to about a 90° angle, and prop it up against something sturdy so you don't have to hold it.

Add a piece of tape near the track pad to secure the upper case and prevent accidental movement.

It's possible to open the MacBook all the way and lay both sides down flat, but this may damage the flex cables and is not recommended.