Real advice:
Ratz Instagib is an incredible game for aim training. Honestly incredible. Look it up.
Aim trainer websites like aimtrainer.io can help with precision and accuracy and both (they're 2 separate things) but personally speaking, it's only for games like runescape and I don't think reflects well in first person shooters. It trains your brain more than it trains your aim.
I highly discourage you for training your aim on garrys mod gamemodes or workshop maps because of the engine, It'd be much better to find some CSGO workshop maps like training_aim_csgo2
(here) or aim/reflex maps with bots like
this
Lowering your sensitivity and DPI, using your whole forearm to aim instead of just wrist, disabling mouse acceleration, increasing the mouse polling rate and having a mouse/mousepad that are comfortable for you are really important. To me, the most comfortable mouse in the world has been razer deathadder. There are some options for interchangable weights on mice that adjust how heavy it is, but it's not really necessary.
But above all, above everything mentioned, just... play the game you wanna be good at. Boxers become good at boxing by boxing, not doing some weird made-up home remedy shit. Play the game, challenge yourself into different situations so you learn how to handle them and that's it. It comes with time. Lots of time.