We start with a simple example :
Inscribe a given right angled triangle (say sides 3,4,5) into a given right angled triangle (say sides 5,12,13), the right angle of the smaller triangle being on the hypothenuse of the larger.
A little more complicated :
Find other locations where the right angle of the smaller lies on another side of the larger.
And then ... Study the general case, that is any triangle into any larger triangle