Heat 4 cups of water in the microwave, or just let it run hot from the tap. It doesn't need to be very hot or boiling (and will be safer if it isn't), just warm enough to help with the dissolving process.
Add ¼th cup of Borax to the warm water, and stir it until the powder dissolves. Let the water cool to room temperature, or stick it in the fridge for a few minutes to speed things up.
Mix 2 tbsp. of glue with 3 tbsp. of water in the separate bowl. For best results, use a small bowl such as a custard cup. Stir until the solution is fully mixed and there are no lumps of thick glue.
Add food coloring to your glue mixture. This recipe will only require a few drops. If making green slime and you only have primary colors, use one drop of blue and two drops of yellow.
Add another teaspoon of Borax powder to the glue mixture; stir well.
Pour both glue and water solutions into a freezer baggie and seal it. Try to squeeze out as much air as possible. Mix the slime by sloshing and squishing the baggie with your hands, but be sure to hold the bag over a large bowl or the sink while you do this in case it breaks.
Open the bag and remove your slime. It will be solid enough to handle, but be sure to wash your hands afterward.