Ok it works

source/funkin/editors/charter/CharterSelection.hx

@@ -1,5 +1,8 @@
 package funkin.editors.charter;
 
+import haxe.Json;
+import funkin.backend.assets.ModsFolder;
+import funkin.editors.charter.SongCreationScreen.SongCreationData;
 import funkin.options.type.NewOption;
 import funkin.backend.system.framerate.Framerate;
 import flixel.util.FlxColor;
@@ -8,6 +11,8 @@ import funkin.editors.EditorTreeMenu;
 import funkin.options.*;
 import funkin.options.type.*;
 
+using StringTools;
+
 class CharterSelection extends EditorTreeMenu {
 	public var freeplayList:FreeplaySonglist;
 	public override function create() {
@@ -26,15 +31,13 @@ class CharterSelection extends EditorTreeMenu {
 						FlxG.switchState(new Charter(s.name, d));
 					})
 				];
-				list.push(new NewOption("New Difficulty", "New Difficulty", function() {
-
-				}));
+				list.push(new NewOption("New Difficulty", "New Difficulty", function() {}));
 				optionsTree.add(new OptionsScreen(s.name, "Select a difficulty to continue.", list));
 			})
 		];
 
 		list.insert(0, new NewOption("New Chart", "New Chart", function() {
-			FlxG.state.openSubState(new SongCreationScreen());
+			FlxG.state.openSubState(new SongCreationScreen(saveSong));
 		}));
 
 		main = new OptionsScreen("Chart Editor", "Select a song to modify the charts from.", list);
@@ -73,4 +76,34 @@ class CharterSelection extends EditorTreeMenu {
 			bg.colorTransform.blueMultiplier = FlxMath.lerp(1, color.blueFloat, 0.25);
 		}
 	}
+
+	public function saveSong(creation:SongCreationData) {
+		// Paths
+		var songsDir:String = ModsFolder.currentModFolder != null ? '${ModsFolder.modsPath}${ModsFolder.currentModFolder}/songs/' : './assets/songs/';
+		var songFolder:String = '$songsDir${creation.meta.name.replace("-", " ")}';
+
+		#if sys
+		// Make Directories
+		sys.FileSystem.createDirectory(songFolder);
+		sys.FileSystem.createDirectory('$songFolder/song');
+		sys.FileSystem.createDirectory('$songFolder/charts');
+
+		// Save Files
+		sys.io.File.saveContent('$songFolder/meta.json', Json.stringify(creation.meta, "\t"));
+		if (creation.instBytes != null) sys.io.File.saveBytes('$songFolder/song/Inst.${Paths.SOUND_EXT}', creation.instBytes);
+		if (creation.voicesBytes != null) sys.io.File.saveBytes('$songFolder/song/Voices.${Paths.SOUND_EXT}', creation.voicesBytes);
+		#end
+
+		// Add to List
+		main.members.insert(1, new IconOption(creation.meta.name, "Press ACCEPT to choose a difficulty to edit.", creation.meta.icon, function() {
+			var list:Array<OptionType> = [
+				for(d in creation.meta.difficulties) 
+					if (d != "") new TextOption(d, "Press ACCEPT to edit the chart for the selected difficulty", function() {
+						FlxG.switchState(new Charter(creation.meta.name, d));
+					})
+			];
+			list.push(new NewOption("New Difficulty", "New Difficulty", function() {}));
+			optionsTree.add(new OptionsScreen(creation.meta.name, "Select a difficulty to continue.", list));
+		}));
+	}
 }

source/funkin/editors/charter/SongCreationScreen.hx

@@ -13,7 +13,7 @@ typedef SongCreationData = {
 }
 
 class SongCreationScreen extends UISubstateWindow {
-	private var onSave:SongCreationData -> Void = null;
+	private var onSave:Null<SongCreationData> -> Void = null;
 
 	public var songNameTextBox:UITextBox;
 	public var bpmStepper:UINumericStepper;
@@ -158,7 +158,7 @@ class SongCreationScreen extends UISubstateWindow {
 
 		saveButton = new UIButton(windowSpr.x + windowSpr.bWidth - 20 - 125, windowSpr.y + windowSpr.bHeight - 16 - 32, "Save & Close", function() {
 			if (curPage == pages.length-1) {
-				if (onSave != null) onSave(null);
+				saveSongInfo();
 				close();
 			} else {
 				curPage++;
@@ -249,4 +249,30 @@ class SongCreationScreen extends UISubstateWindow {
 		iconSprite.setPosition(iconTextBox.x + 150 + 8, (iconTextBox.y + 16) - (iconSprite.height/2));
 	}
 
+	function saveSongInfo() {
+		for (stepper in [bpmStepper, beatsPerMesureStepper, stepsPerBeatStepper])
+			@:privateAccess stepper.__onChange(stepper.label.text);
+
+		var meta:ChartMetaData = {
+			name: songNameTextBox.label.text,
+			bpm: bpmStepper.value,
+			beatsPerMesure: Std.int(beatsPerMesureStepper.value),
+			stepsPerBeat: Std.int(stepsPerBeatStepper.value),
+			needsVoices: needsVoicesCheckbox.checked,
+			displayName: displayNameTextBox.label.text,
+			icon: iconTextBox.label.text,
+			color: colorWheel.curColorString,
+			parsedColor: colorWheel.curColor,
+			opponentModeAllowed: opponentModeCheckbox.checked,
+			coopAllowed: coopAllowedCheckbox.checked,
+			difficulties: [for (diff in difficulitesTextBox.label.text.split(",")) diff.trim()],
+		};
+
+		if (onSave != null) onSave({
+			meta: meta,
+			instBytes: instExplorer.file,
+			voicesBytes: voicesExplorer.file
+		});
+	}
+
 }