This is a shorthand recipe for a 7 stitch wide knit & crochet seedstitch ukulele strap. You do need a strap button on the body to make it work.

The strap can be customized to your needs, by widening and lengthening the band, and should be
pretty easy to adapt to a guitar shoulder strap too.

Materials:
cotton laceweight yarn in 2 colours that go well together, or substitute with DK weight
4 mm knitting needles (or circulars), 3 mm crochet hook, needle to embroider & work in ends

Techniques :
Creating DK yarn out of lace yarn: holding two strands of cotton lace together, make long crochet
chain stitches with length of loops 25 cm resulting in 6 strands held together. Do a couple at a time.
https://www.youtube.com/watch?v=L7mug_y4BZM
Knitting: knits and purls, cast on, yarn over, slip a stitch, M1L increase, K2tog decrease, seedstitch
pattern
Crochet: reverse SC a.k.a. crabstitch, slipstitch in the third loop of a chain
Embroidery: hemming stitch

Disclaimer: this pattern has not been tested, and sadly I can not offer any support on a regular basis.