Buses to Sigtuna run regularly from Uppsala, Enköping, and Arlanda. From Arlanda, it takes about 30 minutes by bus no 579 from terminal 5 (www.sl.se). From Stockholm, you can travel easily with SL Pendeltåg (commuter train) and then bus 570 or 575 from Märsta station (www.sl.se). From Uppsala, you can either go by commuter train + bus which in total takes about 50 minutes or bus all the way which takes about the same length of time. Look for travel options to and from Uppsala on www.ul.se
From Enköping it takes just over an hour by bus (www.ul.se).