Sweep away any dirt or debris from the area
Dig out and remove all the old material that's in the joints with a putty knife.
With a wet-dry vacuum, clean out the joints between the slabs very well.
Apply a bonding adhesive, usually epoxy, into the joints with a brush.
Insert foam backer rod.
Cover and seal the joint.
Expansion joints are virtually never needed with interior slabs, because the concrete doesn't expand that much—it never gets that hot. Expansion joints in concrete pavement are also seldom needed, since the contraction joints open enough (from drying shrinkage) to account for temperature expansion.