I am only like 5' 7" Here is how I do it. Go to a local school or park and get next to a basketball pole. ( or a van or a truck with lumber racks ) You put one pedal up and one down. Hold uni straight up along side of pole and stand on tire with your toe wedged against the frame so it cant roll forward. Then while holding the bball pole you hop up and put the other foot on the down pedal. Grab seat with other hand and in one motion take your foot off the tire and boost yourself up basically shoving the seat into place between your legs and putting the other foot on up pedal . Once seated , push off pole and ride. The tall ones ride really easy. To dismount I usually just stop and grab the back of the seat with one hand and let it fall forwards. you land on your feet and hold the seat so it doesnt get bent.
once you get that dialed in , you can also free mount without the pole with same technique. you just have to go from tire to pedal to seated in one motion. You have about a second and a half. ... I cant do it....
Easy Right LOL.