Nope, not hard at all.
In fact the SickBikeParts shift kit goes together on the engine with surprising ease, in a step-by-step manner with clear and concise instructions.
I had my bike up and running with the kit all attached in a few hours, and never looked back at a single speed setup.
I strongly suggest that you get the "Deluxe" kit as it comes with a whole bunch of useful tools included, and a few optional extras - well worth that little bit extra.
Also, order the "dished 30 tooth" chainwheel gear and the optional 9 tooth Jackshaft gear if you want serious hill climbing ability - these parts are installed on my bike and used with very good effect; parts that i should have bought from the start.
If you need even more hill climbing ability, SickBikeParts offers a 24 tooth chainwheel option.
Below is a web link.
Sick Bike Parts
In the photos, you can see the left and right side of the SickBikeParts shift kit and how it is installed on the bike.
With the low range gears sellected, my max towed weight during a durability test was 75 kilos (165 lbs), going uphill, although at slow speed.