The best glue to use on leather is a vinyl acetate adhesive. Vinyl acetate adhesives are versatile because they are easy to apply, penetrate leather well and dry clear.
To apply a vinyl acetate adhesive to leather, apply the product to both of the surfaces that are being bonded together. Use a bristle brush to apply the adhesive, and place a sheet of paper along the edge of the glue line to ensure that the application is even. The adhesive can also be thinned with water to make it easier to apply. The leather is ready to be stuck together when the adhesive changes color from white to clear. It should also feel tacky to the touch.