I explain below how to get here and visit the historical park by your own but you can also go through this tour to Si Satchanalai from Sukhothai. It will cost you more but you will have a guide, and won't have to worry about climbing onto the right bus, missing your return bus, and you will avoid finding yourself standing on the return bus as I did ;)
See here a list of accommodations close to Si Satchanalai Historical Park
To go visit Si Satchanalai Historical Park, you can also book one of the tours above.