Decide on the general shape and depth to your pond. A garden hose is ideal for this purpose. Simply lay the hose to the shape desired. 

When deciding the shape avoid sharp angles and narrow sections which can create 'dead' areas of water in your pond

Calculating your liner size is fairly easy. Figure out the overall maximum length and overall maximum width of your pond. Measure the depth of your pond. Add twice the depth to both the length and width dimensions. That is the size liner you require.

If for instance your excavation is  3m x 2.4m x .6m deep (10ft x  8ft x  2ft deep) you would add 1.2m (4ft) to both the length and width.

e.g.	m = (3+1.2) x (2.4+1.2) = 4.2m x 3.6m liner required ft = (10+4) x (8+4) = 14' x 12'  liner required