My favorites website for making marshmallows is King Arthur’s

My best tips for making successful homemade marshmallows always have a candy thermometer on hand you can’t do this without it. I suppose you could but the gaging of when the corn syrup and water reaches 240 degrees would be hard to measure.  When you want to add color and flavor to marshmallows an extract would be easiest way to do it with some gel food dye. I like using cocoa powder and hazelnut extract, as well to make cocoa hazelnut marshmallow. Another tip is to make sure you grease and foil the cookie sheet or whatever pan you decided to use to pour the marshmallow into for it to set. It has to sit overnight. When it is set up and you can cut it into cubes you have to grease your knife and I would use plastic gloves to handle the marshmallows and dust the marshmallows with powder sugar and a little cornstarch so they don’t all stick together.  I use a clean bowl with a lid to shake the marshmallow cubes to make it easier. King Arthur’s recipe is always very good. The success rate is at least 100% if you follow directions.