Hello,
I used to repeat Buddhist mantras and read on Buddhism, but then I stopped and practiced Hellenic Polytheism (my alias is because of a priest of Apollo in the Iliad who Apollo loves and treats very well). But then Buddhist elements appeared in my life, and I didn't know why until I read a little on Orphism, the gods are leading me towards Buddhism for a reason.
I would like to attain the Jhanas but I don't have much privacy, and I have back problems, so I'll have to find a really efficient way. Any ideas of how to do it?

Howdy Jon!

Welcome. It's nice to meet you. I'd love to hear about what Buddhist elements having been popping up in your practice to help contextualize it. Id personally say the easiest way for beginners to get good concentration for jhanas would be either to use a mantra or metta meditation. I don't have much personal experience with mantra meditation, but I've had some friends who have had good results with it. For that I'm sure you could find instructions online, and I'd maybe suggest picking a phrase from Hellenic Polytheism that 1. Has significance 2. Resonates with you 3. Embodies a value you want to have more of. The other option would be a metta practice which if you do it over a long enough time will help you develop lots of compassion, and most of the metta meditators I meet are very sweet and nice people. TWIM (tranquil wisdom insight meditation) would be the route I'd suggest, mostly because I've done it personally and have seen it has good results.

For your back issues, I'd suggest using a regular chair. I do it personally and I find it conducive for practice. As one of my teachers once told me "there is no magic in the floor"

​​​​​​​hope this helps!

This is a great entry point for the TWIM stuff: https://www.dhammasukha.org/the-6rs

You can also read this: https://www.amazon.com/Path-Nibbana-Mindfulness-Loving-Kindness-Progresses-ebook/dp/B06WRPZZQF

2.99 on kindle.

The obligatory disclaimer: I highly endorse their practice, find their take on jhanas very interesting, I don't really endorse their world view or takes on morality. 

For soft jhanas try this:
https://ncu9nc.blogspot.com/2020/10/a-quick-guide-to-producing-bliss-with.html

In my opinion the two biggest obstacles are 1) if you are suppressing emotions and 2) if your brain chemistry is not up to the task of producing bliss.

To test for the second issue, try meditating when you are naturally happy, maybe after a big meal that elevates your mood.

For the first issue, let yourself feel any emotion even an unpleasant one just to be sure you have your "emotional gate" open. Often, if we are having some kind of unpleasant emotion like annoyance, anger, fear etc. we try to hide it or keep it under control. It helps us function in social situations but doing it is going to bleed over into positive emotions and it will be hard to experience bliss at the same time.

Also some people report a kind of hangover, an opposite reaction to intense feelings of bliss, so its good to be aware of this and if you experience it, don't over do the bliss next time.

Preparing with this relaxing meditation can make it easier too:
https://ncu9nc.blogspot.com/2020/08/preparing-for-meditation-with.html

I think more in terms of access relaxation than access concentration. The practices that produce relaxation also develop concentration so I am not saying to ignore concentration just that relaxation is important and it seems to me often ignored. When you are fully relaxed you are not feeling any unpleasant emotions, you feel good to start with, and it is much easier to access bliss from that state.

​​​​​​​Awesome !!!

I should have mentioned that this practice is best done lying down so that might be easier on your back, or you can do it in whatever position works best with your back problems, whatever position helps you to relax as much of your body as possible works best.

And I explain how it connects to the jhanas here:
https://www.dharmaoverground.org/c/message_boards/find_message?p_l_id=10262&messageId=26092809

I second using some sort of back support (a chair or lying down). A lot of teachers have very practical advice on the matter - which is that if you're working on concentration, adapt the posture most conducive to developing it. Experiment and see what works for you. My best position is sitting cross legged o na chair with my back supported. 

It's also available for free in the organization's online library, here: https://library.dhammasukha.org/uploads/1/2/8/6/12865490/the_path_to_nibbana__d_johnson_f18.pdf

The good stuff starts are chapter 5 (page 84).