to feel one's way, grope around, fumble
“J’essaierai, bien sûr, de faire des portraits le plus ressemblants possible. Mais je ne suis pas tout à fait certain de réussir. Un dessin va, et l’autre ne ressemble plus. Je me trompe un peu aussi sur la taille. Ici le petit prince est trop grand. Là il est trop petit. J’hésite aussi sur la couleur de son costume. Alors je tâtonne comme ci et comme ça, tant bien que mal.” — I shall certainly try to make my portraits as true to life as possible. But I am not at all sure of success. One drawing goes along all right, and another has no resemblance to its subject. I make some errors, too, in the little prince’s height: in one place he is too tall and in another tooshort. And I feel some doubts about the colours of his costume. So I fumble along as best I can, now good, now bad, and I hope generally fair-to-middling.
From tâter.