get some 2 inch insulation foam, at home depot they come in HUGE like 8 foot by 11 foot sheets, so make sure you have a way to transport it, they are only like 11 bucks so get 2-4 sheets depending on the size of your park, then get some mortar, regular high strength concrete has little rocks in it and you dont want those. also for scooping the concrete i suggest getting those cheap dust masks because concrete is baddd to inhale. so you get home, cut up the foam into your park and whatnot, once you have cut the foam and made the ramps you want get a bucket, mix the concrete, dont make it to watery but dont make it to dry ethier, find the perfect mix, then spread it iver the park, work one area at a time. you wont get it perfectly smooth at first, i learned this the hard way. you lay it down at first, wait 1-2 hours, then smooth it. repeat till it is smooth like butter. then go get an assortment of sandpaper like 80 grit up to 120 grit and sand the round parts starting with the 80 and working up untill your park is smooth. now you can do this but its not required but i highly recoment it. get some spray laquer, the clear kind, and if you want to paint your park paint it before you do this. so you get your clear spray laquer and you evenly coat it all over the park, do a coat, let it dry and then do anohter coat, i like to do 3 coats because i paint my crete stuff with acrylic and it gives it a cool wear away.
so yea thats how you make a park. pm me if you need help