I have purchased some Halloween items. How do I capture ghost?

You click on the halloween item and there should be an option for you to search for ghosts

Once the Halloween Items are placed in your sim's home, simply click on them and select "search for ghosts" and the active sim will proceed to do just that. The activity lasts for six minutes each time you do it.

At the end of the six minutes, the house lights will flash on and off, your sim will have a red bubble over their head. Clicking on the red bubble will release the ghost from whichever item your sim is investigating, you must then click on the ghost three times before it escapes the property. If you manage this, you will have captured that particular ghost.

Make sure it's haunted item and has a little red ghost on it, if you have it, put it anywhere then click on the haunted item, and click search for ghosts