Charge for labour only (no materials)
Bill stitching or alteration work where the customer brings their own fabric.
Create the job
On the Jobs tab tap New Job for the work, but leave the Materials list empty if the customer brought their own fabric.
Use a fixed price
Under Pricing, switch on Fixed price and enter your labour charge (the multiplier needs a material cost, so it won't work here).
Complete it
Tap Mark Complete. Because there are no materials, you'll be asked to confirm — tap 'Complete anyway'.
Bill it
Invoice the job like any other to charge the customer for the work.