Lyse your cells by heating them in a lysis buffer. Although there are several types of lysis buffers, they all contain detergents to rupture cell membranes, chemicals to inhibit the alteration of phosphoprotein levels, and salt and buffers replicating the internal environment of the cell to stabilize protein conformations.
Incubate the lysate in a solution containing an antibody raised to the target protein. To obtain the most reliable results, incubate in a serum containing an excess of antibody. This can be determined by running titration experiments to measure the binding affinity of the antibody.
Incubate serum in a solution containing a secondary antibody that binds to the antibody used to target your protein. This will aggregate the antibodies and your target protein into large clumps that can be easily removed from solution.
Precipitate the target protein by spinning the incubated serum in a centrifuge. Remove the liquid and dry the remaining pellet to remove excess serum.
Detect the phosphorylated form of your target protein. Trying to target only the phosphoprotein using immunoprecipitation is imprecise. A typical method is to dissolve the immunoprecipittion pellet and perform a western blot, a method where protein is separated on an SDS-PAGE gel by molecular weight, transferred to a nitrocellulose membrane and exposed to antibodies to measure specific protein levels.