Place 3/4 of the confectioners’ sugar on top of the melted marshmallow mixture. Fold sugar into marshmallow mixture. Flavoring can be added at this point if desired. Place solid vegetable shortening in easily accessed bowl so you can reach into it with fingers as you are working. Grease hands and counter GENEROUSLY; turn marshmallow mixture onto counter. Start kneading like you would dough. Continue kneading, adding additional confectioners’ sugar and re-greasing hands and counter so the fondant doesn’t stick. If the marshmallow fondant is tearing easily, it is too dry; add water (about 1/2 tablespoon at a time) kneading until fondant forms a firm, smooth elastic ball that will stretch without tearing, about 8 minutes.