This is an absolutely fundamental skill. Stopping & navigating hazards safely has a dependency on being able to quickly & instinctively mount & dismount.
Until you’ve mastered this skill and can do it reflexively, you shouldn’t cycle on public roads.
The practice area should be as level as possible and afford sufficient space to hop along as you mount the Penney-Farthing and also allow you sufficient space to dismount or turn it to dismount.
Although a park might seem the best place to practice the skill, unleashed dogs are common and as a beginner you probably don’t want to add to this to the mix. Also, do not choose a location near a blind juncture where you have no forewarning of approaching vehicles. Much badness, that.
The below videos illustrate the mount technique in close detail to enable you to see how the feet & legs need to move. The first video is a very quick HowTo and the second- longer- video goes into all the granular detail about clothing, suitable location and technique of mounting & dismounting with front and rear views of the procedure.