Bandha

Bandha's are specific muscular contractions and are used to regulate the flow of
pranic energy. We can also prevent prana (life force) from leaking as we use them
together.

Uddiyana-bandha (the abdominal lock) is performed by the contracting and
"sucking up" the abdominal muscles into the cavity and toward the spine.

Use the abdominal lock carefully, the point is not to harden the lower belly muscles.
As you practice drawing the abdominal into the spine you can eventually soften
the lower belly creating this lock. Be kind with yourself if you don't "get it" right away.

Mula-bandha (root lock) is performed by lifting the base of the pelvis. We do this by
refining unconscious contractions of the anus and genitals. Once we can become
conscious we can isolate drawing up only the perineum. Try not to hold your breath.

Mula-bandha is especially useful when jumping into plank, for instance.

Jalandhara-bandha (chin lock) is performed by lowering the chin to a raised
sternum.

Helpful advise

Like everything else, if you don't get it right away don't get frustrated.
Some say it's the journey, not the destination that matters.

More information on bandhas can be found in books like "The Yoga of Breath" by
Richard Rosen.