I was just wondering what the exact procedure is to change a single sink from meat to dairy and vice versa.
Thanks so much
In principle, a single sink can be used, alternatively, for meaty and milky, provided it is thoroughly cleaned before use.
When this is done (in a kitchen that does not have two sinks, one for milky and one for meaty), it is preferable to use water that is lukewarm rather than hot, which solves virtually all potential problems involved. If hot water is used, it is preferable to rinse dishes under the tap, without actually placing them in the sink. If this is not convenient, it is preferable to have two plastic bowls, one for meaty dishes and one for milky dishes, or two flat surfaces that fit the sink (one meaty and one milky), on which the dishes are placed.
The advice above is given to avoid any potential problems, which can occur if solids/fat remains in the sink from previous use, if the pipe gets blocked, and some other issues involved. Due to such issues, it is therefore the common custom to have two sinks, and to have two bowls or surfaces for a single sink. If, for whatever reason, this custom cannot be followed, it would generally be permitted to use the same sink for meaty/milky after previous use, provided one ensures that the sink is thoroughly cleaned before use.
Sources: Although the sink might absorb taam from food it directly comes into contact with, its next use would involve nat bar nat de’heteira. This should be avoided lechatchilah, but is permitted bedieved (see Shulchan Aruch and Rema, Yoreh De’ah 95:3).