>Is this just because I'm a beginner and I need practice?

practice cant hurt.

>How can I be SURE I'm writing the right equation for the problem?

you have the right equation for the problem if 1.) you can get an answer 2.) that answer is correct (usually easier to verify if you end up needing 500 grams of copper for instance you know you've done something wrong)

if you cant verify for reasonableness try using the same setup for an easier scenario that you can figure out easily (ex. making copper with copper and an alloy, your equation should tell you you need copper and no alloy)

>Am I doing the wrong steps?

you really need 2 or 3 equations here. in this case i would first find out how much silver i need, how much alloy i need to have have that and then the rest is the copper, which is 3 equations.