I'm sure it is
entirely possible to have sex in the situation you describe. Trick is, you have to understand
how!
Hang on! Isn't sex supposed to be "plug and play"? Isn't it self-evident?
Well, the fact is, no! And incidentally, my definition of "having sex" isn't limited to putting your penis in your girlfriend's vagina, but includes a lot of other things, commonly but deceptively called "foreplay"; more correctly described as "important
parts of the sexual experience" without which women in particular, don't find it all particularly exciting (but often go into "actress" mode to soothe the expectations of their fellow).
In case you don't see where this is heading, it's all about "lubrication", and whether the lady is
ready to receive you. If in particular she is young and has not had intercourse before (which strongly implies that she has not delivered a baby and in the process, been "stretched"), she may well be anxious about the matter, resulting in a complete lack of the lubrication that she
should have to receive intercourse, combined with a degree of tightness of the vaginal muscles that may
completely prevent entry, condom or no, foreskin or no.
The broad answer to your query about "how someone with phimosis/ frænulum breve can have sex using a condom" is - that's most likely the
only way. How to put it on? Simple - when you have an erection, you hold the rolled condom over the tip of your penis, and unroll it over your penis. That's it - that's
all of the instructions, as long as you understand the concept of unrolling it over the penis (and get it the right way round in order to do so - it won't work the other way round), you've got that part licked. And speaking of that, I understand some ladies - particularly those with a commercial interest - even develop the knack of doing that "unrolling" of the condom, with their mouth!
So are you now confused about all this bit about "pulling the foreskin back" first and "rolling it forward again"? Don't be - it is an invention of those in circumcising societies who don't know anything about foreskins.
No, it's the
next bit that is important - we're getting back to
lubrication. The condom is on your penis, and you want it to go into the lady's vagina. It needs to be
lubricated to do so. Now either she is
enthusiastic about that happening, in which case you will have found out when you touch her vulva, because it will be warm, wet and slippery - and you certainly
should for a variety of reasons,have been stroking her vulva (do I need to explain?) - or she is clearly
not ready, and not lubricated. If that latter
is the case, and you choose to proceed then all is not lost, as when you obtained the condoms, you also purchased their "partner" - a tube of "personal lubricant" an amount of which (it
was kept in your inside pocket so it got warm, wasn't it?) you now put on the outside of the condom or (figure this out) into her vulva.
There will now be
no friction along the sides of the condom; the end will press its way into the vagina against her modest muscular resistance. Having no friction is important as the same degree of "drag" as could pull your foreskin back, has the potential to stretch and rupture the corresponding part of the condom. As Jim suggests, your foreskin will move in much the same way as you do when masturbating which is nothing more nor less than what you would expect anyway - notwithstanding some people's mythology that your penis can "feel" the difference between masturbation and intercourse, the only actual difference is the more total enclosure of the penis by the vagina (and of course, all the associations of contact with your partner).
End of sexology lecture.

Please tell me if I have covered your points, and are my presumptions that this is the first experience for both you and your girlfriend, correct?
Now can I just mention something? I keep saying to readers in general that they need to read up on old "threads" here, but I only realised whilst looking up something for a previous thread, just how fast this forum "moves along". Each index page lists
ten threads, you click "Next>>" down at the bottom right corner to go to the next older index page of another ten. You will note that Jim has locked all older than the last two pages worth, simply because it isn't practical for him to keep checking the old ones for additions. So if someone has something to say much later, such as their success in correcting their problem, they need to post it as a new thread and include a
pointer to the old one.
Now when I say you need to read "a few pages back", I mean you really need to read
every thread from each index page, then every thread from the next, and so on, for something like six index pages. Yes, that's sixty threads but really, they're not long and
you're the one wanting to learn about it - there's no reason you shouldn't have or spend the time and if you do that, you really
will learn a lot about the subject and about your particular problem as it compares to what others have discovered.
Jim and I already know about it - we've read it over the weeks, months and years, and it really is a bit of an "ask" for us to go back through it just to find something to explain to each new enquirer which we already have to someone else and indeed any others. To you on the other hand, it's all pretty much new, and all
important knowledge. Even if you see someone who has
not succeeded, we will have made it reasonably clear why that would happen, and hope you can learn from that as well.
You will in particular find answers regarding why it does or does not matter whether you have a tight frænulum in terms of having sex, as well as the discussion on whether it matters at all how you put the condom on, and why.
It is
particularly pleasing when people do come back and report success and gratification - not (just) because it makes us feel good to know that we've succeeded in helping someone not only to fix a problem, but to have confidence in their own abilities to deal with such things and to
learn rather important things about how their body functions - but because it becomes a "testimonial" to others who are unsure as to whether what we advise
really will work in their "particular" circumstance.
Of course as you must know, you can't believe everything you read and the Internet could be amongst the worst examples of this - unless you are lucky enough to "trip" over this or one of the other more honest sites, you might well believe that circumcision (or frænuloplasty) is necessary. How on earth can you tell? Well, you evaluate what you read against what you know already, what you observe in "real life", how things actually "work". Anyone who asserts that your foreskin (or frænulum) is "magically" different from any other part of your skin, had better have a really good explanation to back it up.