Press J to get keyboard (arrow keys) and mouse control.
Click and hold to charge attacks. Down arrow also works. The arc (when it even shows up) displays the arc your guns will be thrown at. Surplus guns seem to be stored, but storing too many seems to make Mami control even worse.
You start in Mami's apartment.
The lower of the two X's in the upper left is a barrel smashing stage. Surprisingly durable compared to enemies.
The upper X is an area leading to Gertrud. Press and hold Up to jump, and then air jump if you're too far from the gate still.
Inside, there are 3 large grief seeds. Otherwise, your objective is the elimination of enemies. If Kyubey takes a liking to your head, just tap the mouse and he'll get knocked away. If he's not on your head, he's usually either chasing grief seed drops or your guns.
And then you get to Gertrud and the bad game design turns into so much lag that you literally can't do anything except get hit (and get knocked completely off the screen).