how to use only some letters

[philsmd]

Unfortunately, I can't do more than link you to the wiki page that explains it very well https://hashcat.net/wiki/mask_attack (what I mean is, I can't read or understand it for you, just spend some minutes and you will get it! you can even use hashcat's --stdout parameter if you want to play around).

hint: --custom-charset2 qwertyasdfgzxcvb123456789 --increment --increment-min 6 --increment-max 10 ?2?2?2?2?2?2?2?2?2?2