"excuses are like azzholes. everyone has one."
are you talking about filing the flight plan or about doing the NAVIGATION from scratch?
if you did not receive schooling on the navigation, you better postpone your xcountry until you complete ground schooling on that subject.
now, from scratch.
1. you get all available charts.
2. check for NOTAMs and FUPs for active airspaces at the time of your flight to avoid them or know the rules of crossing them.
3. you select your turning points.. considering your training status, lonely road junctions, single mounts or other significant terrain features.. i'd suggest large towns but there usually you can expect reasonable traffic there.
4. you check for the weather along your route and at destination and select appropriate diversion / alternate.
5. from there, you will know how much fuel you need. calculate estimated elapsed time considering the worst predicted wind (try for several flight levels, even though for general aviation cessna style it's pretty much the same like for helicopters.. stick to ground if flying against the wind, pop up if flying with tail wind to mitigate/ use the winds aloft.
remember your route fuel does not end at destination but at ALTERNATE plus holding time.
6. nav aids. check them, make sure you can use those and you know the equipment onboard to use them.
7. write down frequencies in order of contact.
8. calculate bingo fuel (starting from alternate, counting backwards so that you know how much fuel you HAVE to have at each turning point).
now you know the fuel required for flight. add persons onboard, cargo, luggage, stuff and check against temperature and runway length. do you fit the MTOW for given meteo conditions? if not, re-route or plan refuel down the route.
that's about that.
been doing this in the cargo bay of helicopter in flight, including plotting the VOR bearings and IFR stuff.
then of course, file the flight plan and keep your weather info updated.