* the neutral vowel. The most common sound in the English language is the schwa, /ə/. Most English speakers aren't even really aware of it - it's the first vowel in
about, the second vowel in
lemon, something that English speakers who don't know phonetics might transcribe as "uh". We use it all the time, in almost every word with 2+ syllables. We also often pronounce prepositions and modal verbs with the schwa. It doesn't exist in Spanish, but English speakers tend to use it unconsciously a lot. This relates to stress timing vs syllable timing. Look it up if you're not sure what I mean - basically in English we time our speech according to the words that are most important, but in Spanish every syllable has equal timing. If you use the schwa and apply English stress timing to Spanish, you'll sound like you're mumbling and people will often have a hard time understanding you.
That is the single most important thing for English speakers learning Spanish, and by far the most common problem I see regularly. Everything else I have here is window dressing - but I'll note it down anyway, as it might be useful.
* single
r in Spanish should see your tongue tap on the roof of your mouth. If you have a rhotic accent it will be easier. If your accent is non-rhotic, you'll need to practise a lot, because it can feel very awkward, particularly with words that have
r before another consonant, or with words that end in
r (like...every infinitive in the whole language
).
* double
r or
r at the start of a word is trilled. Again, this depends a bit on your native accent - I grew up in one of the few parts of the English-speaking world with a trilled
r so I find it easy, but I know a lot of English speakers really struggle and have to practise it a lot.
*
d in Spanish is formed with the tongue touching the back of the top teeth (in English the tongue is further back). That's why
Madrid will often sound, to English speaking ears, more like
Madrith.
*
v and
b are identical, and more like an English
b. People will understand if you pronounce
v like in English, but it's a big thing to work on if you want to reduce your accent (I would say it's sort of like a German saying
v instead of
w in English - we all understand but it's an instant tell).
*
ll should have a distinctive
y sound. I often hear English speakers saying, for example, "
tortía" when they want to say
tortilla. Make sure you are vocalising the consonant sound.
* syllable stress is a LOT more important in Spanish than in English:
vino is totally different from
vinó. People make mistakes here all the time. Active listening is crucial, along with emulating the stress the other person uses.
* the vowel sounds in Spanish are simple because there are so few of them, but they're hard for native English speakers because the exact sounds don't exist in English. I've come to terms with the fact that I'll never master them perfectly...but one thing we can master is keeping that their pronunciation regular. So for example most English speakers will pronounce
Granada with the exact same vowel pattern that we use in
banana, with the central
a being notably different. In Spanish all 3 of the syllables are completely regular, with the same vowel sound.
I was using it over summer to start learning French and it kept asking me to say “are you a horse?” A totally normal conversation starter, non? 
I switched back to books after that but I still quite like the mini podcasts on DuoLingo if you’re subscribed. They’re quite handy for learning pronunciation!
It's exactly the kind of advice I need at the moment. I've definitely been struggling with the double 'r' at the beginning of words. It's a sound that's completely unfamiliar to me. And yep, I would pronounce tortilla as "tortía" 
Think I've finally got the hang of syllable stress - listening to lots of Spanish music/native speakers helps with that.
