For a pillow I'd probably make a template, for a quilt I use the Tri Recs Ruler to make life easy.
here is a demo o the ruler,
https://www.youtube.com/watch?v=HQCdh-ZkRAM
and a demo of fitting them together so they come out right
https://www.youtube.com/watch?v=aTPE1EFZHpc
and if this isn't an isosceles, sorry
