If possible, this is done in a real bathroom, with an actual, full-immersion bath. But often it's not possible, and in those cases using a bowl of water in a private space is entirely appropriate.
If possible, the area should be decorated in accordance with the season or the celebration, to help put the celebrants into the right frame of mind.
You also need a candle, some incense, some salt, and some oil. It's best if these are appropriate for the occasion, too. A beautiful splash can make all the difference for someone who is still caught up in their daily affairs, and help them to leave them behind for a little while.
The person preparing the splash fills the tub (or bowl) with warm water. (Remember, people will be climbing or reaching into it. They probably won't want a cold bath!) Then light a candle, and turn out all other lights. Stand for a moment, and let the cares of the day dissolve. Then ground, and center, and raise a small circle around the space by facing each compass direction in turn, and inviting the Guardians of that Quarter to join you. (This is done in much the same way that the larger circle is raised, but usually more informally.) When all are present, ask them to help you form a circle, and trace the form of the circle in the air, turning clockwise, while visualizing a bright light coming from the end of your athame or your finger. When you reach your starting point, visualize it joining, and flowing up and out to form a sphere, with the splash at the center of it.
Then light the incense from the candle. As it begins to smolder, pass it three times over the water. Then stir the water clockwise (deosil) with your strong (dominant) hand, while saying;
Symbols of fire and air, fill this water with your brightness, will, and knowledge, that all who come into the Circle this night shall be made bright and clear, strong of will, and ready to obtain and share all knowledge.
Next, take three measures of salt, add them to the water, and stir deosil with your strong hand, while saying;
Symbol of earth, fill this water with your holiness, patience, and wisdom, that all who come into the Circle this night shall be made holy, full of patience, quiet and calm of spirit, and ready to obtain and share all wisdom.
Next, take the oil, and sprinkle a bit into the water, once again stirring deosil with your strong hand while saying;
Sacred oil, symbol of the spirit, fill this water with your sacredness. Let all who are troubled of spirit find soothing peace, as oil calms the water, that all who come into the Circle this night shall be made serene, calm, and full of peace, ready to share in the communion of spirit with each other and the Gods.
Finally, stir the water itself deosil with your strong hand while saying;
Blessed water, symbol of life and love, be filled with all holiness, that all who come into the Circle this night shall be clear and holy, serene and joyful, full of patience and love, that we may worship the Gods, and find peace, contentment, happiness, and companionship this night. May we be filled with knowledge and wisdom, that we may walk the path we were always meant to walk.
Then lift your hand, and make a pentagram over the water, while saying;
This blessing I make, and gifts I ask, in the names of the Gods; for the good of all, and by the free will of all! So Mote it Be!!
The splash is now ready. The person who prepared it should use it first, and then each of the other celebrants in turn, until all have splashed and are ready to join the circle. The last one should blow out the candle.
After the circle is over, the person who prepared the splash should return to the room, thank the Guardians, take down the circle, and clean it all up.
These words are only suggestions, of course. Feel free to tailor them for your own situation, or celebration. (Just be very sure that you know why you are changing whatever you change.) This isn't as formal as the full circle, so relax and speak to the Guardians and Gods as your friends!
Herbs can also be added to the water, additional candles may be floated in it, etc. Just remember to keep little things like safety and cleanability in mind. Remember, we are practical!
When it's your turn to use the splash, go into the room, close the circle behind you, ground and center, and use the water to bless yourself. Use whatever words you want, and draw whatever symbols you feel happy with on your body with the water. Splash a little of it wherever you feel it would be appropriate. Just remember that others will be using it, too.
If the splash is in an actual tub, make sure that you have taken a regular bath or shower before you go to the place the Celebration will be held in. The reason for this should be obvious!
When you are finished, cut yourself out (remember, you are in a Circle) and tell the next person that it's their turn.