There are several theories that exist in the idea of "planning" the gender of a baby during the conception stage. To be completely honest, most theories carry a very low accuracy rate, and many theories are considered "old wives tales." It would be helpful for you to talk to a medical expert or even a midwife or doula to at least get an idea from someone who may have more experience in this field and can provide you with some statistics.
One of the most popular theories that people use to plan the gender of their next child is called the "Shettles Method." This method is based off of the idea that sperm carrying the male chromosomes (XY) both move and die faster than those carrying female chromosomes (XX). The Shettles Method suggests having intercourse as close to ovulation as possible in order to increase the chances of conceiving a boy, and several days before ovulation to increase the chances of conceiving a girl.
Another theory, called the "Whelan Method" of gender selection, is similar to the Shettles Method. The Whelan Method utilizes tracking your basal body temperature (which goes up slightly when you are ovulating) and suggests intercourse 4-6 days prior to the rise in temperature for a boy and 2-3 days prior for a girl.
Note: in almost all "gender selection" theories, the accuracy percentages are so close to 50/50 that most medical experts agree they cannot be counted on as the reason for having a boy vs. a girl. In short, you always have a 50/50 chance of having a boy or girl, therefore, the statistics are mostly inconclusive.