If you wanted to extrude 100mm of filament at a feedrate of 20 mm/sec you could enter the commands below. Remember that you should zero your extruder first before telling your extruder to move. This is because your extruder could have a very high position in mm, for example 10291mm. If you then tell the extruder to move to the absolute position of 100mm. The extruder will have to travel 10191mm backwards, and that will take a long time! You can also turn on relative extruder moves with M83, and then you would not have to zero the extruder as it just adds 100mm to the position the extruder is already at.