How do I generate a Snort pcap file?

I am new to using snort and still learning in university. I am wondering after I find an intrusion how can I log it and save it as a pcap file? What would the syntax look like to do this? So I can analyze it further with Wireshark. I am doing what the guy is doing in the video below with two virtual machines. This is for an at home lab. Here is the video, https://youtu.be/iBsGSsbDMyw