To expand on Paul's response. Since everyone else's is so dang complicated. But his was lacking enough detail.
Step 1: Remove the 3-way switch from the junction you want to remain and (Assuming this is the hot source) replace the switch with a single pole switch. (continued on step 2)
Knowledge 1: 3-way switches only have 1 hot (the black). Then two wires that run through to a second switch to the circuit that are called travelers.
Knowledge 2: At the power source switch, the whites are tied together and capped. Leave them like that.
Step 2: Place the black into one of the hot poles (gold)
Step 3: Identify the traveler (white or red) and attach it to the other hot pole. (mine was red)
Step 4: On 3 way switch being removed, take the chosen traveler (mine was red). tie it together with the power traveler (mine was black)
Knowledge 3: You can trial and error what you tie together as long as you make sure the power on the input. Is seperately attached.