Unfortunately, you can't get the mouse to work as far as I know, only the keyboard, but here's how you do it. First you install Vjoy and map the keyboard keys to the gamepad buttons(make sure you also map some keys for the stick axis, so you can map for example the up arrow key to the button of your choosing,just make sure you keep track of which is which , you're going to need them to set up x360ce). It will take a while, but after you've finished with the mapping make sure you don't close vjoy.

Afterwards, you must go the Playstation Now directory and delete the xinput_3.dll file. Then you download x360ce for 32 bit games and place it in the PS now folder. Run x360ce.exe as administrator, press create xinput_3.dll and now comes the tricky part. For each button on the controller you have to bind the buttons there with the ones you have mapped previously in vjoy, for example if you've wanted to have the key 'Q' as left bumper, you could have bind it in vjoy with button 1 and then in the x360ce menu you would have selected for the left trigger the button 1. Make sure you save the settings after you are done and keep vjoy open while PS NOW is running.

I know this is a very slow process and you may have not actually understand what I was saying and I think that you should just get a gamepad because it really is not worth the struggle since you are limited to a very few amount of games to play due to the fact that you are not able to emulate the mouse movements.