Play either the D major scale or the D major pentatonic scale (which you may notice is just like the major scale but fewer notes).
Have a few goes through then try (at your own speed) with the D major backing track - click on the track in the list and it will start to play (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