If you go for the crimping method use a good crimping tool (not the $5 ones) and the proper diameter crimp connector. The connectors come in different sizes identified by colour. For car stereo’s buy some blue and red ones and just try with one fits best. For the tool you have the ones like in the picture and Ratcheting style tools. The Ratcheting style tools are easier for a novice user (harder to get wrong). After you made the connection pull firmly on the wires to make sure it will hold.

For soldering there are tools using lighter gas, not to expensive and very practical. Insulating the soldered connection is best done by a heat-shrink tubing (available at electronic parts stores), not the tape as shown in the picture on the crutchfield website. As the state in the text, tape may dry out and fall off and since it is behind the radio you will only notice it if fuses start to blow or smoke comes out of you dashboard.

For both methods you have to take some of the insulation of the wires, be careful not to damage the wire while doing this.

Rob