There are several bikes scattered around Goatsville, some green and some red. Approach one and you can hit
to jump on it. Take a moment to appreciate seeing a goat riding a bike and the re-focus on the achievement. :)
You will push the
back and forth to control speed, and right/left to control leaning. It is very sensitive so do not overdo it. Also pushing forward will increase your speed greatly, so try to avoid doing that.
I found it much easier to be going very slowly, barely pushing forwards and focusing on staying mostly upright.
I also noticed that it seems to be okay to be pushing against a wall or other stationary object, which makes it considerably easier to stay upright because you can push the
hard forwards and stay upright with minimal left/right adjustments.
Also note that this achievement only pops once you fall or get off, hence why I was pedalling for a couple minutes before ultimately crashing and popping the achievement. I genuinely had no clue how long I had been on the bike, so I recommend a stopwatch of some form to avoid frustrations.
Also, users n1ckKz and Calex dEUS recommend you enable slo-mo to make this easier.