Installing the tankless water heater does require some expertise in assembling gas and water fittings. While installing pex is reasonably easy, these two taskes are certainly not in the realm of your everyday DIY'r.
WARNING: Not doing the gas piping right could be very dangerous and in many jurisdictions requires a certified professional to do the installation or at least test it BEFORE first operation.
But there are a few things you can do to make the job easier.
1) Use Pre-fab Pipe
Most of the big box home improvement stores will have all the black pipe materials in various pre-cut sizes and lengths that you need for this job so doing any cutting of pipe can be avoided or eliminated entirely. The stores will have all the pipe, values, nipples, T's, safety values and other parts. JUST BE SURE TO GET GAS RATED PIPE AND FITTINGS!
While black pipe is adequate for the gas pipe I recommend getting either stainless or galvanized for the water lines.
2) The Gas supply line
You will need to get and route the appropriate size (diameter) gas pipe from your outside source point to the hot water heater. The perfect pipe for this is the corrugated stainless steel tubing (csst) as that manufactured by Gastite. The material bends around corners similarly to PEX so you can run one continuous pipe without elbows. The only connections to be made are at the outside wall (to connect to your gas source) and at the water heater. THESE TWO CONNECTIONS ARE CRITICAL SO ONLY PROFESSIONS SHOULD MAKE THESE.
Using this corrugated pipe eliminated any of the work associated with running and connecting the black pipe normally used for the task.
3) Use the right tools
You simply must have the right tools to tighten these connections. A good set of pipe wrenches is essential.
4) Use plumbers putty
All the fittings must use the appropriate gas-rated putty for sealing the connections.
5) Testing is Absolutely mandatory
You need to test all the fittings for leaks using a soapy fluid.
Installing Propane Gas at the Cottage
11 rules for venting a tankless water heater (gas)
3 reasons to use corrugated stainless steel tubing (csst)