It started with collecting stones, setting them up on Mount Ebal (on the western side of the Jordan River), covering them with plaster, and writing down this whole body of instruction.
Deuteronomy 27:2-4, 8 When you cross the Jordan River and enter the land the Lord your God is giving you, set up some large stones and coat them with plaster. Write this whole body of instruction on them when you cross the river to enter the land the Lord your God is giving you—a land flowing with milk and honey, just as the Lord, the God of your ancestors, promised you. When you cross the Jordan, set up these stones at Mount Ebal and coat them with plaster, as I am commanding you today … You must clearly write all these instructions on the stones coated with plaster
Joshua eventually got around to doing this after taking Jericho and Ai (see Joshua 8). It was early in the invasion. Like a coach preparing his team, God was setting up a good foundation for this business of making a new nation.
Then, once the stones were in place, there was to be a big celebration featuring sacrifices and feasting:
Deuteronomy 27:5-7 Then build an altar there to the Lord your God, using natural, uncut stones. You must not shape the stones with an iron tool. Build the altar of uncut stones, and use it to offer burnt offerings to the Lord your God. Also sacrifice peace offerings on it, and celebrate by feasting there before the Lord your God. You must clearly write all these instructions on the stones coated with plaster.
When it happened, the Canaanites would have had to have been watching all this and wondering what on earth this invading army was playing at. What sort of army loses its momentum and any element of tactical surprise by loitering around religious ceremonies mid-campaign?