Put the glycerine amount that you plan on using in a clean gallon bottle or sealable container. This makes mixing easier. It also makes determining the percentage much easier, and you can even eyeball it with a good amount of accuracy.
Fill the rest of the container with demineralized water.
Seal the container firmly, making sure that it doesn't have any leaks.
Shake the container vigorously. If shaking is difficult for you, you can stir the contents with a spoon or rod. Make sure the contents are combined thoroughly before you use the mix.