The following procedure is explained in Getting Started.
You start by creating a file events.txt that contains the photon data files you downloaded. Note that in this case there the Fermi LAT data server generated eight files, each only about 1 MB (mega-byte) large because there are only few events above 10 GeV.:
$ ls -1 *_PH??.fits > events.txt
$ du -hs *_PH??.fits
968K L1309071835220B976F4330_PH00.fits
1004K L1309071835220B976F4330_PH01.fits
1.1M L1309071835220B976F4330_PH02.fits
1.2M L1309071835220B976F4330_PH03.fits
1.0M L1309071835220B976F4330_PH04.fits
1.1M L1309071835220B976F4330_PH05.fits
1.0M L1309071835220B976F4330_PH06.fits
840K L1309071835220B976F4330_PH07.fits
808K L1309071835220B976F4330_PH08.fits
Now tun the following commands in sequence. gtselect will just take a few seconds, gtmktime a few minutes and gtltcube will take a few hours ... so we suggest you copy the file gtltcube.fits from the solutions folder so that you can continue quickly
$ gtselect infile=@events.txt outfile=gtselect.fits \
ra=INDEF dec=INDEF rad=INDEF tmin=INDEF tmax=INDEF \
emin=10e3 emax=316e3 zmax=100 evclass=2
$ gtmktime scfile=../../spacecraft.fits evfile=gtselect.fits \
filter="DATA_QUAL==1&&LAT_CONFIG==1&&ABS(ROCK_ANGLE)<52" \
roicut=yes outfile=gtmktime.fits
$ gtltcube evfile=gtmktime.fits scfile=../../spacecraft.fits \
outfile=gtltcube.fits dcostheta=0.025 binsz=1
On my laptop gtselect takes 2 seconds, gtmktime takes 4 minutes and gtltcube takes two hours!