Buy approx 50-70cm of each (112cm 0r 150cm wide) fabric, one colour for the outer, and one as a liner. I like to use quilters cottons as the range of colours/patterns is much nicer, but any non-stretch fabric would do. The plastic can be found in the tablewear section of stores like Spotlight and Textile Traders and costs around $6 or $7 a metre. Sometimes its hard to get hold of so if you are planning in making a few bags it pays to buy up big when you see it in stock somwhere.
The general idea for cutting out is to cut a large rectangle and then cut out identical sized squares from each corner. The size of the square determines the depth of the bag. For my example above, the overall rectangle was 52 cm X42 cm and the little squares that got cut out were 11cm x 11cm. Sewing with a 1cm seam allowance this resulted in a finished bag with the dimesions of 30cm wide, 30cm tall and 20cm deep.
Once you have worked out the desired dimensions cut 2 of the lining, 2 of the outer fabric and 2 of plastic. The handles are should be about 8 to 10cm wide. The length can vary depending on if you want to sling the bag over your shoulder or not. 60-80 cm if you want a shoulder bag or 50cm if you want them to be carry handles. Cut 2 handles from fabric (I like to use the same fabric as the outer bag fabric) and 2 from plastic.
Sewing the lining...
NOTE... if you want pockets on the inside of you bag you should attach those to the lining before you begin sewing it together.
Crappy photo I know, but on one of the side seams leave an opening of about 20cm- this is because in a later step you will need to turn the bag right side out. Dont be tempted to make the hole smaller than 20cm, the plastic is hard to get through anything smaller.
This is a tricky part to describe. The little sqaure cut outs are now pulled apart so that the botton and side seams can meet. Sew the seam to create the corner of the bag. I hope the picture helps you understand what I wrote. I am not even sure that it makes sense!
Thats it for the lining.... now you need to make the outer shell, the handles and then join them all together. Part 2 is here http://thetinwhistle.blogspot.com/2007/12/tutorial-plastic-coated-bag-part-2.html