Your first snapshot

using AwesomeAssertions;
using Nullean.Snapshots;

public class GreetingTests
{
	[Test]
	public void Greets() =>
		Greeter.Greet("world").Should().MatchSnapshot();
}

There is no verified file yet, so the test fails and says where it put what it got:

Text snapshot is new: no verified file at tests/App.Tests/.snapshots/GreetingTests/Greets.verified.txt
Received: .artifacts/.snapshots/received/App.Tests/GreetingTests/Greets.received.txt
Accept:   snap accept App.Tests/GreetingTests/Greets   (or: dnx Nullean.Snapshots.Cli accept App.Tests/GreetingTests/Greets)

Read the received file, then accept it:

snap diff Greets
snap accept Greets

1. what would change
2. write it as verified, delete the received file

A key can be given in full, as any unique suffix (Greets, GreetingTests/Greets) or as a glob (GreetingTests/*). Or walk through everything pending at once with snap review.

You can also accept while running tests, locally only:

SNAPSHOT_ACCEPT=1 dotnet test --project tests/App.Tests --treenode-filter "/App.Tests/App.Tests/GreetingTests/Greets"

tests/App.Tests/.snapshots/GreetingTests/Greets.verified.txt is now part of your tests. Review its diff like code: when a later change alters the output, the test fails, the received file shows the new output, and accepting it is a decision you make, with the change in front of you.

Name them, or the second call fails because both would use the same file:

html.Should().MatchHtmlSnapshot(name: "header");
json.Should().MatchJsonSnapshot(name: "payload");