These guys are using sharp handsaws. I have a Stanley Sharptooth saw. It is important to ask whether you know the proper technique for using a hand saw. I didn't until this year. There is a definitive technique for cutting square and straight with a hand saw. Start by laying out your cut lines, either in pencil, or scoring. The saying is, you never want to cut what you can't see. So you should never be cutting blind. To cut the shoulder, use your saw to start your saw groove on top. This is pretty easy because it's on top and you can see the whole top cut line easily. Saw a 1/16th to 1/8th inch groove. Now get up on top of your piece and saw the same type of groove on one of the vertical sides, so you can see the whole line very well, making sure not to cut past the your marked lines. Once you've established this "guide groove", You can begin sawing on the of the top corners, using the grooves as your saw guide. Saw until you connect the lower corner of the side with the nearest corner of the top. Then move to the other side of the timber, so you can stand over the next line to cut and saw much the same as the other side, connecting the far corners. Once you've done this you can place your saw in the top groove and saw with the saw teeth level until you reach the required depth.
