Well, initially you take a deep breath, gulp slightly and smile, saying it will be fine, of course I can. Then you have a couple of days of serious self-doubt and find anything else to do rather than tackle this knotty problem!
But thanks to the internet and a wealth of photographs of chocolate labradors, as well as a couple of mediocre extra photos of the spaniel pup, you bite the bullet and give it a go and find to your surprise that it all goes rather well.
Next up, a portrait of a black spaniel with some superb reference photos to work from!
N.B. Yesterday the lady who commissioned this came to pick it up - as soon as she saw it she burst into tears! She was still crying an hour later apparently - I think I got it right!