Try the C minor pentatonic scale (including 'blue' note or flat fifth) a few times on its own first, then play along after selecting the C blues backing track (which starts after one bar rest and a count-in).

UKULELE:  C major   G major   D major   F major   C blues   G blues   D blues   F blues

UBASS:  C major    G major    D major   F major   C blues   G blues   D blues   F blues