1s
2s 2p
3s 3p 3d
4s 4p 4d 4f
5s 5p 5d 5f
6s 6p 6d 6f
Now draw arrows from right to left on a diagnol.
The first arrow will go through 1s
The second arrow will go through 2s
The third arrow will go through 2p and then 3s
The fourth arrow will go through 3p and 4s
Etc. Keep that pattern going and you will understand the order inwhich each subshell is filled. Hope that helped! Although it is difficult to explain without being able to draw it.