Step1: The first thing you do is draw 45° lines where you want your mitre joints to be. If you have a drop saw or sawing block you should not need to draw the lines.

Step2: Now either cut along the lines with a hand saw or drop saw but remember if using a drop saw you can usually adjust it to a 45° angle and if using a sawing block it should have a 45° slot for mitre joints.

Step3: After the joints have been cut check them to see if they are accurate and then join the joint together with an adhesive and leave it to dry. If the joint is not reasonably accurate or if it is cut at the wrong angle the ends will not fit together properly.