Congrats!
Check out the UI & Macro forum for more info on macros, especially these 2 forum stickies
Tons of very knowledgeable people there.
Beware castsequence though, it can be glitchy at times and often results in trading effectiveness for convenience. Like if you need to re-apply agony during it’s pandemic you have to go through 3 seconds worth of other spell casting to put it on and that can result in missing it’s pandemic window.
Hold the alt key and activate the macro to cast Siphon Life, hold either the ctrl or shift key and cast corruption when you activate the macro, otherwise the default is agony (assumes that alt, shift, and ctrl are not keybound to other actions which would eat the “modifier” key).