Well, here's one guy's approach...FWIW:
...but maybe not the way I'd approach the task. There are other DIY videos in a search that will be of help.
I wish you well on this endeavor...not only the initial production of the track, but the periodic maintenance thereof in the future...somewhat dependent upon your climate, seasonal changes, critters, insects, etc., etc..
Materials used...rail, ties, 'spikes', all need to consider the environment...even in a 'dead rail' application. There are suppliers of brass, aluminum, stainless rail, but not on the cheap...IMHO. Ties/sleepers?...rot-resistant...cedar, redwood, treated, non-wood. Spikes?: Weather resistant/proof. Etc., etc., etc..
Making through track can be the 'easiest' task...switches/turnouts will be another story. Switch "kits" are available from a few suppliers, but, again, not on the cheap.
It would also be helpful to find someone who's already gone this route for outdoor trackage and pick their brains...and recommendations. Hopefully locally, but maybe a forumite will volunteer assistance.
Viel Glück!
KD