Any time you do something intentionally you are using your will. To therefore strengthen the will is the same as strengthening muscles - through use. There are literally unlimited ways to use the will, but as Ra says, the start of them is all with the focusing of attention.
Think of a circle, you are using will.
Engage your breathing, you are using will.
Stretch your arms above your head, you are using will.
Visualize a room, you are using will.
Sing out loud, you are using will.
Sit still, you are using will.
Stir up emotions within yourself, you are using will.
The key is intention which directs the attention. Discipline is the effort to make the intentions conscious.
Think of a circle, you are using will.
Engage your breathing, you are using will.
Stretch your arms above your head, you are using will.
Visualize a room, you are using will.
Sing out loud, you are using will.
Sit still, you are using will.
Stir up emotions within yourself, you are using will.
The key is intention which directs the attention. Discipline is the effort to make the intentions conscious.