Let’s go to the gym. The bass-finger gym that is!
This work out practices one-finger-per-fret with elaborate string crossings. I have plenty of tips for you to succeed if one-finger-per-fret is hard for you. As always, this only works if you stay relaxed, move as little as possible and keep your fingers close to the strings. Keep breathing and give it a go.
Youtube’s slow down function (lower right, click the gear wheel and select a slower speed) will come in handy if you want to work your way up to it.
This handy PDF lays it all out in score and TAB. For your own personal practice, break this down into small bits.
Remember: short spurts done more often is better than long ones and long breaks in between. Ready? Set? Go!