Back sweetening is just adding sugar for sweetness after fermentation has finished or you have stabilized the must. With a mead, I would back sweeten with only honey, and preferably the same honey used in the recipe.
There are some things to be aware of.
For starters, it is easier than you might think to add too much honey and end up with syrupy mead, and there is no way to fix this. To avoid this, I usually experiment by adding teaspoons of honey at a time to a cup of mead until I like the result, and scale the amount to the total volume of mead I want to sweeten.
Honey is very hard to measure, mix, or pour when cool or cold (which can contribute to the above problem), so I like to warm the honey to ~90º F by setting the container in hot water. Then I dissolve the desired amount of honey into a small amount of mead (about the same as the amount of honey I am adding), trying not to introduce too much oxygen, and then pour this mixture into the aging vessel.
Honey can affect the clarity, causing a slight haziness. I don't care if it tastes better, but some do. This might age out, it might be cleared with fining agents, or it might not matter to you.
Time is needed to blend/mellow the flavor of the fresh honey. This does not take as long as mellowing a "hot" alcohol flavor, but don't expect to sweeten and serve without noticing a clear honey flavor (also depends on the honey). Days or weeks could do it.
