1. THREAD — six letters
Trace THREAD through these cells: 1 at row 4, column 4; 2 at row 5, column 4; 3 at row 6, column 4; 4 at row 6, column 5; 5 at row 6, column 6; 6 at row 5, column 6. Reading along that sequence gives T, H, R, E, A, D without skipping a step.
The movement is down twice, right twice, and up once. That creates three straight segments separated by two turns. The path spans three rows and three columns, visits four edge cells, and includes one corner cell. Once these six letters are assigned, 26 open cells remain for the other three entries.