Have a good look
HERE - there are good links to videos etc. Personal tuition is to be preferred but if you study the vids and reads the advice notes etc it is not difficult to pick up the basics. As Aber says, the scythe needs to be sized and set up to suit an individual. I read somewhere that "a scythesman(woman) would no more lend their scythe than their false teeth". Serious back/muscle ache is more an indication of a badly setup scythe or poor technique. "Let the TOOL do the Work..!!" It is not a slashing tool - more a Micro-toothed saw. Do not try to cut too wide a swathe until you get more proficient. Concentrate on finishing each sweep so that the cut grass etc is left in a windrow clear of any standing crop and work methodically. If you "hack" at it, you will waste more energy, cut less grass and risk breaking the snath (the stick bit). The most important tip I can offer is to carefully check the area for any obstacles, particularly of the rock or metal variety. Whetting (sharpening) is a fairly easily acquired art but repairing a damaged blade is more difficult as I am learning after "finding" some partly buried wire fencing netting left by the previous owner..

Fortunately, I bought a range of blade sizes so I can still mow.
I find the scythe infinitely better than a screaming/whining engine. You can enjoy hearing the birds and insect life and feel and smell the breeze without being "all swaddled up". Mind you, at times, some bug protection is desirable. That said, it does depend on how much you need to mow at any one time which is why I am negotiating purchase of finger bar mower and hay rake. With 9 or so acres with some heavy rush area, a single scythe just ain't going to cut it.