Perhaps you need to repair a leather vest. More likely, you want to add a cloth patch or insignia (like your motorcycle club's colours) to the vest. The most secure way is to sew the colours onto the vest, with a heavy-duty needle on a sewing machine. Ironing with patch adhesive will ruin your vest; instead, you need to "cold iron" your patch using a liquid adhesive, which will remain flexible and keep the edges of the patch from peeling.
Lay the vest on the ironing board so you have a flat surface to work with. Smooth out the leather to minimise wrinkles.
Position the patch on the jacket where you want it. Trace a few of the edges with the crayon or wax pencil so you know exactly where to place the patch when it is glued.
Pour the leather adhesive into a cup. Use the brush to spread a thin layer of adhesive over the patch, nearly to the edge. Dip the brush again and add a slightly heavier coating of adhesive to the outer edges. Feather the brush by flicking it over the edges, to ensure that you have covered them with adhesive.
Lay the patch on the vest, using your tracings as a guide. Use a spoon, or a cold iron for a large patch, to press the patch to the leather. Start at the centre and move outward until you have pressed the entire patch. It's likely that some adhesive will squeeze out of the sides of the patch.
Wet a few cotton swabs in water, then dab them quickly in the paint thinner. Squeeze out most of the liquid, then run them carefully over the edges of the patch. This will remove the excess adhesive and any wax marks.
Allow the vest to dry fully per the adhesive manufacturer's instructions---typically eight hours.
Sew a few anchor stitches on the extremities of the patch (like the tips of the wings or the corners of the banner) to ensure that the ends never peel. Smooth leather will require a heavier layer of adhesive than suede.
Do not use a heated iron, even on its lowest setting. This will shrink the leather, causing it to pucker. Do not use any adhesive other than a leather adhesive, which will not damage the leather.