This is probably the most important achievement in the game because it sets up almost every other achievement. Having a save file with a functioning 100,000 population city will allow you to clean up the monuments and a couple miscellaneous awards. I do not normally post solutions, but this seemed like an iconic achievement in the game and this website does not have a guide yet.
Above is the link to a YouTube video where a guy speed runs all of the milestones in an hour and a half.
If the link stops working, look up "City Skylines Two Rivers Speed Run"
This guy was a record holder. You are not. Please watch the video at half speed and pause often.
Some important details: The guy in the video moves a lot faster than you will. He will take out a lot of loans that you do not need towards the end. (The beginning ones might be necessary) Also your landfills will fill faster, meaning that you might have a trash problem where he does not.
To avoid issues of timing, try to keep your population around where his population is. Your city has a pause button and so does the video so try to keep everything in sync. Last note, his traffic techniques on the interstate do not need to be copied perfectly. Anything close will get you to the 100,000 eventually. The video stops at the last milestone (80,000) but your city will naturally keep growing as buildings level up.
Hope this helps a little. The achievement is difficult, but this guide should get you close :)