Just play the game? You don't lose anything if you just play casual matches to learn. The only attachments that need any real practice/adjusting are scopes. The other attachments all usually make the gun easier/better to use so if you practice with a naked gun and get good with it, then just know the attachments will make it easier. By the time you reach level 40-50 you should have already tried basically every gun/attachment (this does not take very long)
And for the abilities that needs a bit more practice but just watch videos the skills are all very straight forward and simple, the only thing you need to practice with them is knowing when the right time is to use them, however practice won't help with that anyway because you don't have a squad or enemies.
