The starter point is your player current values and your current training setup given by intensity and stamina share.
From there you can define for different ages a different stamina target and the tool will calculate the lowest stamina share necessary to reach it.
The period to correct the stamina value to the target value is given by "Trainings to correct stamina" and the tool will try to make the correction before the target age if possible.
For example: in case you have a target of stamina at level 8 (excellent ) at 21 years old you should then input 21 (years) / 0 (days) / 8 (stamina target) and then click on "Calculate".
You will receive a full stamina share plan detailing every different stamina share value necessary to reach and keep the player at the stamina target level. It will be always the lowest necessary stamina share.
You may also inform to the tool that the need of a stamina target ends for example at 21.111 hitting "Add new" and inputting the new step as 21 (years) / 111 (days) / 0 or blank (stamina target). The tool will fallback to your original stamina share one day after the target age.
Finally you may define a new target as for example 7 (solid) at 29.000 hitting "Add new" and inputting 29 / 0 / 7 as the new stamina target.
You may add as many steps for stamina target as you need. Always keep them in age crescent order and remember to hit "Calculate" to receive a new stamina share plan.