Make the laser base and the bullet, then get it to slide from side to side and fire the bullets in the correct "space invader" manner, IE only fire one if the previous one was destroyed, this has the advantage of not only being true to the original, but it`s easier to code as well
Then make the shields (from several thin objects stacked together) and make it so that the player can shoot out parts of them by deleting the thin objects the bullets collide with, after that you want to add just one invader and get him to march from side to side and approach you at the end of each row.
After that you can make hime fire back at random intervals and then add a bunch of mates to keep him company, then finaly add the random bonus saucer and the score displays, and thats it, just tweak it so that it gets faster for each level completed, search the net for the original sounds and gfx (they are out there...I did a clone a couple of years back), and thats it, one copy of invaders, add in a line that will display "coded by vertexrexx" whenever you press a key and you can show it to your mates and when they say "you didn`t code that" then get them to press key x
Mentor.
Get Pro sorted, I am a customer not a BETA tester.