Sealing is the cheap part. The clean and the re-sanding underneath it are what decide whether the driveway still looks right in three years.
The process
We pressure clean the surface and the joints, treat any efflorescence or organic staining, re-sand with polymeric sand, then apply sealer once the surface is fully dry — usually the following day.
Timing
Sealing needs a dry window and surface temperatures in a workable range, which in the GTA means roughly May through mid-September.