The materials you will need for this are: (see above picture for images)
• An AC 120 volt to DC 12 volt transformer or AC 120 to DC 18 volt transformer, these are the two main power supplies for cordless drills, however if your drill says another voltage then get the appropriate transformer for that drill. Basically you need a power supply that changes 120 volts AC into your drills voltage DC. ( This is about the only thing that will cost you money assuming you already had the drill or got it for free)
• Screwdriver to disassemble the drill
• Cordless Drill (obviously)
• Electrical tape
Step 1: Take Apart Your Drill
Step 2: Cut the Wire Between the Trigger and the Battery Terminal
Step 3: Cut the Plug Off of Your Transformer and Strip Wires
Step 4: Splice the Wires
Now that your wires are stripped to around the same length, cross them in an X form, where the center of the x is about midpoint of the 2 stripped sections of wire
Whichever wire is on top (closest to you) bend away (or back) and then down, the wire will make a upside down L shape around the back wire.
Images 3 and 4:
Now take the other wire, bend it towards you (or forward) and then down, twisting it around the first wire, making a M shape
There are two directions to twist, you do not want to twist the entire wire, just the splice. I recommend using your thumb and forefinger of each hand to twist the wire. One wire will go clockwise (or towards you) the other wire will go counter clockwise (or away from you), pinching hard, twist only the spliced section, slowly moving towards the insulation with both hands if you are using solid core wire, or thick gage wire once you get near the ends, I strongly recommend you wear gloves or use needle nose pliers to finish up the twist if you got close enough to the same length of stripped wire, and made your X close enough to the center of the two wires you should end up with both ends of the wire running out just at, or before the start of the insulation and a super strong splice.
Wrap electrical tape around each set of wires individually being sure to cover any exposed metal so that it will not touch the metal from the other wire and then tape the two bundles together to make a clean, finished, and insulated job that will not short out.