I've been practising mindfulness for 10 years now on and off. I am so much better when I am doing it!
I have several mindfulness books and this one is good for this purpose:
The Mindful Way through Depression by Mark Williams, John Teasdale, Zindel Segal, and Jon Kabat-Zinn
The authors are credible and it also includes a CD for guided meditation which I find helpful especially when energy is low. I also found it a really helpful discussion about how the depressed mind works and how you can work with it.
Also recommend Mindfulness for Women by Vidyamala Burch - she is a British Buddhist who (I think) runs a mindfulness programme in Manchester supporting people with chronic illness/stress. (She has other books with that focus). I haven't read this one but have some of her older books and I really like her voice.
Fo me it was worth investing some time in reading more in depth books because it increases my motivation to practise over the longer term.
The practises I found most helpful are the basic ones - body scan, mindfulness or breathing, and loving kindness. Body scan bring your attention back to your body, mindfulness of breathing is about attending to the present moment (and whatever arises in your mind at that time, without baggage). Loving kindness is about compassion, for yourself and others. I find that this is really important because it supports my attention to my thoughts being kindly in nature - as when depressed my tendency is to think critically especially of myself.
My preference would be to go for books that focus on these core meditations rather than those that guide you to think 'about' different images etc.
A classic book about lovingkindness meditation is 'lovingkindness' by Sharon Saltzberg.
If you are seriously interested though I would really recommend learning through classes. I attended a buddhist retreat centre weekend course on mindfulness and loving kindness meditation and it was a brilliant way to learn the technique so I could practise at home, they also check things like your posture and so on that can make a difference to your practice, and they explain some of the barriers you're likely to come across and how to deal (boredom, fatigue, wanting every meditation to be nice, etc.).
Thanks for posting your message. I was about to post a general upset wail, and instead you've prompted me to re-engage with my own practice! Good luck