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 Above left  leather covered  copy of; The Perfume of Books,  by Erika Mordek,  a book on old bindings, so it deserves a full leather binding with a sort of  Cambridge Panel,a trade binding pattern from the late 1700's..  But  during the blind tooling an over hot tool burned the leather.    What to do now?
One specialist suggests making a feature out of the mistakes, another suggests burn them all and they will match.  But I think I must try to repair, but I have never done this before..

 So  above right,  I start with  a tracing paper of the area. The tooling is a double line at the outside, a single line on the inside and a roll with pattern between.The upper diagonal is a double line.  So I think if I cut out the burned leather, and inlay a replacement, and retool, the lines will hide the repair. So I trace the tooling lines  to make a cutout jig Left below,   and shown sitting over the burnt leather.

                             
 
Seems like a good way to get the piece I need, but after cutting a few pieces I decide the best way is to prick the tracing paper onto the leather and cut the replacement piece with a slight angle, holding the leather firmly with ruler pressure.. The cut the leather burnt piece out the same way prick the corners and use a ruler to cut straight lines with a slight undercut.  Leather was then dyed to match.

Here is the cut out of the damaged leather and the pieces matched to reinsert.

Above,the pieces well pasted and inserted with care to remove any traces of the paste. Then Below, put under light weights to dry.  And below right  awaiting the courage to have another go!