Baking soda is the easiest way.
If you are running a small system, Reef Success CALK is a great way to add KH and Calcium at the same time.
I am using it and it is great.
You simply dump the powder into the sump twice a week and your done. No premixing or settling whatsoever.
Well priced to for the amount used.
Then you simply add a little bit of Baking soda to keep the kh up to around 10KH. IMO.
I only use this system as my system is too small to use a reactor.
Otherwise a reactor is the ultimate for stability.
(The KH buffer in the CALK is not sufficient to keep it all the way up, only buffers it a touch, but sends the calcium up hugely).
I love it!