fertilizing my geranium plant
I'm assuming you want to keep the geraniums inside as a houseplant over winter - there are other ways to keep geraniums into the next season, described in the references below. If that's the case, the ideal growing conditions may be challenging: bright light but not too warm (60's). Fertilize once a month with a typical houseplant fertilizer at 1/2 rate. Don't overwater. Then in late March or early April, start fertilizing more regularly and at full concentration. But don't expect blooms until May or June.
Read here about wintering geraniums, including other methods, such as taking cuttings or storing dormant plants: