Summarize:

Tie one end of the band or string to the base. Secure a knot of the string near the edge of the base. This will create a permanent knot that is useful for fitting the capo to your guitar. Loop the other end of the elastic band or string around your base. Detach the loop before attaching to the guitar. Put the base across the guitar neck at the fret desired, and loop the other end of the band around the other end of the base. This will secure the base over the strings. Make sure it is attached securely. Depending on what elastic band you use, you might need to double loop it to increase the pressure. The pressure needs to be high to effectively press down the strings across the fret. Take it off and tie a knot on one end of the band to make it tighter. Repeat until the strings are pressed firmly against the strings. Test the capo by playing a chord or a few notes. When you test the capo, play chords in the open position to test if the open strings sound good. The open strings are the notes that are affected by the capo.
Connect the band to the capo. Connect the capo to the guitar. Readjust the tightness.