Sinj sits about 30 km northeast of Split on the D1 road, an easy half-hour drive. Frequent buses connect the two; the trip costs a few euros and runs roughly hourly through the day, less often at weekends.
The nearest airport is Split (SPU), with cheap European connections especially in summer. From the airport, the easiest option is a taxi or a hire car — there is no direct bus, so you'd otherwise need to travel via Split centre.
By car, Sinj is on a natural route between the coast and the inland — it is one of the gateways to Bosnia-Herzegovina via the Kamešnica pass.