The additional portion indicated by the square brackets is only played first time.
If you do not manage to play line 24 (25), stop at the last high line which you can play, but retain the low alternations from page 5.
Second Study (page 8)
Take line 37 for example:
First play it like this:
By practising like this you will learn to "place" and "centre" your notes correctly and to improve the fluency, flexibility and intonation of your playing.
For warm-ups see my book "Warm-ups + Studies" (Editions Bim, CH-1630 Bulle/Switzerland).
In no. 31 of Brass Bulletin (September 1980) we shall look together at some more ways of improving our practising of these excellent exercises by Herbert L. Clarke.