The following tips assume you are right handed. Reverse these instructions if you are left handed.
Clear a good space on your table and in regard to chairs and stuff on the floor around you. The table height can affect your accuracy/comfort. Too low and your back will ache, too high and you will not be able to put enough downward pressure on the ruler and it may slip. A kitchen worksurface can be a good option for some people.
Fold your pressed fabric selvedge to selvedge (I pre-wash all my quilting fabric on a cool gentle cycle, with just a small amount of detergent). Place the fabric well onto your cutting mat and line up the folded edge on a horizontal line towards the bottom of your cutting mat (the selvedge edges are at the top, so way from you). The majority of the fabric should be to your left and the raw edges at the right. Make sure the fabric is fully on the mat and supported on the table - not dragging or hanging over the edge of the table.
Take your 24" long quilters ruler and line this up with one of the vertical mat lines, positioning it so that you will be trimming off just the ragged raw edge. Make sure the ruler is fully supported on the mat and not half off it as this will not help you make an accurate cut. Cut the raw edge of the fabric off, making sure you walk your hand up the ruler to support it throughout the length of the cut (and not getting what I call a curved 'banana cut'). Your fabric is now 'square'.
Pick the fabric up and reverse its position on the mat so that the cut edge is now at the left side. You are then ready to cut strips at the width you require - making sure to do all the above steps so that your fabric and ruler are well supported and that you make accurate cuts.