Like the above - cock, lever, shoot. However, I have one of the early (2 digit SN) Spencer's in 44 Russian so I also have to lean the rifle at 45 degrees to the left to facilitate chambering!
True story - the last time I used it, I was striving to finish a stage in less than a minute. At the fifth or sixth stage, I accomplished the feat! Now, mind you, as this was an ongoing quest during the match, a lot of pards were watching to see if I could do it. When I got to the unloading table, I regrettably found TWO loaded revolvers! As the revolvers were last, I completely forgot about them in the heat of the moment. But in addition to my forgetfulness, the spotters and TO also failed to notice that I had not shot them! Only one person in the posse noticed and I heard him comment as I was unloading.