It's very similar to doing vertical scrolling, but for H scrolling you use the $2000 and $2400 nametables. And switch the 'nametable address' bit in the $2000 register every time your scroll rolls over to zero, to alternate between the 2 nametables that are being used.

Another helpful bit in the $2000 register is the 'increment address by 32' setting. You'll probably want to use that when doing H scrolling, that way you can draw a column of tiles by simply doing consecutive VRAM writes (for every 8 pixels or whatever that are scrolled).