Since you already have your pond liner you must have designed an area for the pond and have it outlined in your yard.
I recommend using a garden hose to outline the design of your pond as it is flexible enough to create flowing curves, "solid" enough not to make squiggly curves (like string or rope would do), and heavy enough that it won't blow away.  You can use chalk or spray paint to mark the outline and while you're at it,  outline your bog and marginal shelves, too (if you don't know what these are, I recommend going through our Pond Informational Course.)  Once you have your design made,  you can start digging.  If your pond is large, start digging from the middle and work your way to the edge.  Dig  to your desired depth and put the dirt in a wheel barrow or other container and haul it to your waterfall area or wherever you planned.   By working from the middle out you make it easier on yourself so that you can have the wheel barrow next to where you are digging and solid ground to transport it.  For those with a small pond I recommend digging your pond in the same fashion but you will have to decide what is best.  Dig to your specified depth including bog and marginal shelves, and the end opposite of the waterfall,
if planned, dig the pond slightly deeper to help create better water circulation and flow to where your pump will be.  If you plan on having a bottom drain, design and prepare for that as well.  Dig your edges of your pond at approx. a 20° angle to
give support to the side walls. 

While you are digging out your pond, take into consideration that you should put 2" of light sand onto each layer to help prevent rocks, roots or debris from puncturing your liner.  After you have completely dug your pond out, level each of the shelves and thoroughly go through the entire pond and remove any rocks or debris that may be protruding.  Add approximately 2" of sand inside the pond to act as a "cushion" and protector under your pond liner. Lightly sprinkle the sand with water and tamper it down.  We also recommend the use of a geotextile underlayment for under your pond liner as well, which helps protect the liner from any rocks or sharp objects.   By adding the sand and underlayment, it also helps to provide protection and "cushion".

Once everything is leveled, dug properly, bottom drains installed (if desired) and sand and geotextile underlayment are installed, it's time to put in your pond liner!  Open the liner and with the assistance of some friends or family members, carefully drape the liner across the pond making sure it overlaps and has equal distance on all sides.  (Your pond liner will be larger than your pond when you drape it over.)  Secure the edges with rocks or bricks and start to fill it with water.  

By the way, you don't have to walk into your pond and manually contour the liner to the edges, the water will do that while it is filling.  Try to have any folds or wrinkles out when you start to fill it.  While the pond is being filled, you can tug and move the liner a bit to work out any folds that develop.   Once the liner is taut, you can reposition the rocks or bricks at the edges
allowing for the liner to sink down and fill with water.

Take some pictures of your pond during different stages of creating it and while the pond is being filled.  You'll enjoy looking back at the photos and have a "brag book" to show your friends and family!

Once the pond is filled with water, you can trim the excess liner off.  If you calculated the size of your pond liner accurately and positioned the liner correctly, you should have a little excess liner that needs to be cut off.  You can cut the liner relatively easy with an X Acto knife or sharp scissors.  Depending on how you decided to landscape the edge of your pond, you can dig a small trench around the pond and bury it with dirt or you can hide it with coping stones, boulders, etc.  One important note to consider when creating the pond is the edge of the pond and landscaping around it.   Do you want a very pronounced and defined edge with bricks or coping stones or slate?  Or do you want a natural border with cobblestones, rocks or boulders?   The edge of your pond helps defines it's "mood" as well as the landscaping around it, so give it some thought.  You will need to decide these things while calculating the size of the pond liner, in case you need extra liner.   Regardless of what material your pond's edge will be, make sure that it's edge is at least 10% higher than the ground around it.  This will help prevent water from rain or sprinklers from going into your pond and any chemicals or fertilizers in the soil around it.  If you have your pond in part of a natural hill, you may have to create a trench on that side of your pond, fill it with pea gravel and sand, which will help divert the water from your hill from going down into your pond.  In this case, you may need a professional landscaper with experience in creating ponds to do the work for you.  Once again, definitely have the edges of your pond at least 10% higher than the ground around it.  You can easily do this with the dirt that came from digging out your pond.  This edge must be level too, or else you will have water leaking from the lower areas.

One important note to make about putting cobblestones or rocks around the pond's edge.   The cobblestones, no matter how much you rinse them off with a hose, will add a lot of dirt to your pond and you'll be so disappointed especially after seeing the crystal clear water in your pond from the garden hose.  You will have to put your liner in, fill your pond up with water, add the cobblestones to your pond and then drain all of the water out, rinse off the rocks even more, clean your pond out and then refill the pond.   This is a big step and can be very time consuming and cause your water bill to go up, but you have to do it and in the order which I described.  The cobblestones have to be cleaned and this is the only way to do it (unless you have a small pond and want to wash each cobblestone off in a bucket of water!)   Naturally other rocks, coping stones, etc. should be cleaned as well.