There are several methods, depending on the the types of 1)inputs on the subwoofer and 2) outputs on the amplifier (aka receiver). You can only use the type that is on BOTH.
Most sub woofers have "line level" inputs (they are the RCA phono style jacks or other style jack) that are marked "In" or "Input" and may have speaker level input & output terminal that accept speaker wires.
Amplifiers or "amp" all have speaker level outputs and many also have jacks that are labeled "Subwoofer" or "LFE" (low frequency effects) Output. If both your sub and amp have the "line level" jacks, then all you need to do is get a cable that is long enough to connect the two together, and then enable the low frequencies to be sent to the sub instead of the normal speakers in the receiver's set up menu. Done!
If you have only a speaker level connections common to both the sub and amp, you'll most likely need to disconnect the Main front L and R speakers from the amp and connect to the sub's Speaker Output terminals - and connect a couple of new speaker cables between the sub's Speaker Input terminals and the amp's Main front L and R terminals.
If you don't understand completely, provide the make and model of the amp and the sub, and I'll be able to clear up anything that isn't clear.