As soon as you're able, purchase the metal detector by pressing the
button and the use
to select the Upgrades menu. The metal detector beeps when you are close to a stash and beeps faster the nearer you get to it. Note that you need both upgrade points and money in order to buy upgrades. You get upgrade points by completing missions, races and finding stashes.
Some stashes are out in the open and some are in homes or stores. You will need to break into the homes or stores to get those. Additionally, some require you to enter a four-digit code to open a safe. Clues to the safe code are usually found on a piece of paper near the safe's location, but using a guide that includes the safe codes is much faster.
The map in the walkthrough lists the locations and safe codes needed so I won't recreate it here. I highly recommend printing out the map so you can check off each stash as you find it. Keeping track of what you've found during the campaign will save you from having to go back to ones you've already found.