Sometimes it's important to make sure that you and your space are cleansed. A lot of times this is done at the beginning of a ritual to begin to make sacred space, but other good times to cleanse a space would be if people have been sick in a house, if things feel spooky, there has been anger thrown around, the space feels stagnant, or often as part of house care.
Times that it's good to cleanse yourself would be if you've felt off, as part of grounding and shielding, as part of ritual, on the full and new moons, and the easiest time, whenever you shower.
When cleansing, the most important part is not the cleansing itself but what you're going to put in it's place. If you cleanse and don't add anything else back in then you're creating a vacuum and you'll just suck back in what you've cleansed or worse. So always go into a cleansing knowing what you're going to be putting in place of whatever you're removing. It can be as simple as "love and light" or something more catered like "poise and confidence" before an interview.
To cleanse yourself the two easiest ways would be to a) take a shower, while standing in the water see all the stagnant, old energy that is no longer serving you wash down the drain, or b) take a bath, and see the water (and herbs, oils, stones, salt, if you've added anything to the water) purifying every part of you. You can layer it and add to the bath water things that will fill you with whatever you want to add in after the cleaning like rose quartz for love, or epsom salts for added cleansing power.
You can also cleanse yourself with incense, you can use a stick of incense (or have a friend hold it) and have the smoke waft all over your body and all the way around you, using your minds eye to see that energy break off of you and be swept away in the wind.
To cleanse a space or an item you can cleanse it in any of the 4 elements or for an extra powerful cleanse you can use all four.
Earth: Bury the item in the earth, or salt (for a room you can sprinkle salt on the floor)
Water: submerge or spritz the item with water (this can be water that has sat in the light of the full moon for extra oomph)
Fire: Very Carefully! you can wave the item over the candle (for a room you walk around holding the candle)
Air: Use incense and waft it around the room or the item through the smoke.
You can adapt any of these methods or layer meanings as you see fit. Now go forth and be clean.