B'sha'ah tovah!
There is a custom to immerse at some point during the ninth month. There is no set time. We have not been able to trace the source for this custom. Presumably, its meaning is to reinforce the pregnant woman's commitment to these halachot and her status as tehorah going into childbirth.
Immersing for this reason does not entail a halachic change in status. Therefore, beyond being clean prior to immersion, the extent of preparation is up to you. You may perform preparations as usual, but need not be as careful or as thorough, nor must you take as much time. No blessing is recited in this case, although you may wish to utter a personal prayer.
Do be sure to explain to the mikveh attendant that you are immersing for segulah purposes.
As this is not a commonly accepted custom, unless you have a specific mesorah (tradition) to follow it, you are not required to immerse.