So I’ve been offered a PhD position at a great university and fully funded, but I’m starting to worry that I won’t be able to cope with it. The techniques that will be involved are something I haven’t done before so I don’t really know if I’ll enjoy them or be able to do them? How much support and training did people get at the start?
I’m worried I’ve only accepted it because I can’t find a job and I know it’ll help with a career further down the line, is this a terrible idea?

The typical level of support you get at PhD level is 'not much'; partly this is reasonable (you're expected to be a capable learner who is steadily becoming independent), sometimes this is unreasonable (disappearing supervision). The realistic expectation is a bi-weekly 1 hour meeting with a supervisor and feedback on written work around the same timeframe.

I think it important to add a PhD will not necessarily help with a job down the line; unless the job you envisage is an academic one. This is not obvious, since as a student you've possibly been through 18 years of being told grades and qualifications are everything, but this ceases to be the case in a lot of postgraduate circumstances.

Negatives out of the way; you can always quit at any point and if you're not enjoying it there's nothing stopping you applying for jobs and being able to pick and choose (since you have a fallback). It might be a PhD is not for you (or you could find you love it), but you're in a better situation having something paying the rent than not. A mistake I see early-career academics (or employees) make is a misguided sense of loyalty to their employer; like they're doing something wrong if they look for other opportunities whilst in-post. For people with an ounce of experience, this is a normal thing to be doing and not traitorous/unfair/unreasonable, and whilst you might think the University and supervisory team are having sleepless nights if you quit, the honest fact is in the grand scheme of things it's next to nothing.

Give it a chance; the things that make a PhD a good experience are often not the topic or your ability; but the lab/group environment and relationship with supervision. These are impossible to guess at until you start.

And I would add that based on whatever information they have about you, they obviously think you're capable of learning the techniques. Funding is very competitive, especially at some universities. They wouldn't award funding to a candidate they thought wasn't up to it. Congrats by the way :)

You can try it to see if you are capable of getting a PhD.
Basically, you learn everything on your own. Professors can provide guidance and the question of whether you will be able to get a PhD depends on your ability only.

Also, have you done a masters before, or an undergraduate research project? It is very much like that, only all your time is spent on the research rather than it just being one module or component.

I don't know what field you are in but I would hazard to say that most PhD students do not know the required techniques beforehand. Some people may be lucky and already know it but the vast majority of students need to learn techniques. So don't worry about not knowing the techniques already, everyone else is/was in the same situation. You will learn most relevant methods through trial and error, where you learn what you need as you go.