Couple tips.
1. You most likely will sound crap when you first start trying, it's just how it seems to go, it doesn't mean what you're doing is 'wrong'. In fact it might well be you're on the right track, you've got to practice to give time for your vocal cords and stuff to adjust.
2. Keep an audio diary, maybe once every night or so record how you're feeling and whats been happening, play it back, see how you're progressing over time.
3. Don't push yourself too hard. You want to get into a pattern of talking as feminine sounding as you can without it actually being too hard on your voice. Try to keep it so you can talk in full volume as in normal speaking conversation.
4. Don't give up, it may well take time and in many ways I don't think there's any particular 'trick' to doing it. It's not that it's within your capabilities to sound great right now, it's that you need to practice so with time you will start to sound better. So even if it feels like you're missing something or not doing something right. It doesn't mean you're not on the right track.
Over time you'll hopefully start to hear yourself sounding better than you would have been capable when you first started, and after that you just start talking like that more and more and it hopefully will gradually smooth out.
Best of luck.