how to hook up old macbook to projector

You're welcome. HDMI was common by 2015, so if your projector is from that time as well, it may have an HDMI input. If so, you're all set. If not, see what kind of inputs it has and we can figure out an adapter.

You can also use an Apple TV and AirPlay.

Apple TV HD 32GB - Apple

The Apple TV uses an HDMI connector, so your projector needs an HDMI input.

Hi,

If your projector has an HDMI input, just an HDMI cable to go from your MBP's HDMI output to the projector.

Depending on the age of the projector, you may need an HDMI to VGA or DVI adapter, too.