Clone the gene in a plasmid that encodes an antibiotic resistance gene (neomycin: neo). If the plasmid DNA does not have this gene, you can co-transfect another plasmid with the neo gene. (Transfection is the introduction of DNA into a recipient cell and the DNA's subsequent integration into the recipient cell's chromosomal DNA.) Importance of an antibiotic resistance gene is that after transfection, cells that don't have the plasmid DNA will die, and only cells that have it will grow in antibiotic-steeped growth media.
Grow the cells that need to be made stable to log phase. This phase is when the number of cells in a culture medium increases exponentially.
You need approximately 1 million cells per transfection. These must be adherent cells, which are those that attach to the tissue culture dish.
Plate 1 million cells in the 10 cm tissue culture dish 24 hours before transfection.
Next day after plating, transfect 10 micrograms of DNA into the cells using the transfection reagent.
Seventy-two hours after transfection, use trypsin, an enzyme, to remove the cells from the tissue culture dish (also called "trypsinizing"). Replate the cells in six tissue culture dishes. The growth media should have G418 antibiotic for neomycin selection.
It will take nearly two weeks for cells to grow. Keep changing the growth media with G418 or other type of antibiotic.
Once, the cells have reached the maximum number that can grow in the tissue culture dish, trypsinize and replate them in dilutions. By using a suitable antibody, you can harvest some of them to confirm the presence of the specific gene of interest.