Proposing to Shirou is a simple affair. Taking him out to dinner at a nice place, getting down on one knee, opening up the ring box. "Emiya Shirou, will you marry me?"
"Y-yes, yes I will!" Shirou rubs the back of his head and mumbles something suspiciously like, "I thought I was supposed to be the one to propose..."
If he wasn't overjoyed, Issei would thwap him for that right now.
Archer requires a bit more thought than that, by which more things can go wrong.
In the middle of a meal that Issei was supposed to cook and Archer took over halfway, with candles that the wind keeps blowing out around their table set-up in the courtyard and bugs flying around, Issei sighs heavily and mutters something stupid like, "this is not how I wanted to propose."
Cue a look that has much less surprise in it than Issei would have liked. "I heard that, Issei."
Issei drops the ring box on the ground and fumbles for it for a few seconds before Archer picks it up for him. "Will you-" they say at the same second, before Issei just nods.
The meal is delicious, as always.
Rin and Issei have been dating for years before anyone even brings up the marriage idea. It's Shirou. Of course such a stupid idea comes from Shirou. They immediately glare at him until he shuts up.
It takes two more months before Issei actually thinks of marrying her for real.
They accidentally run into each other buying rings to propose with. Since that's convenient, they propose there.
Shirou is granted the rare privileges of being their best man and saying 'I told you so.'
Diarmuid is a traditional kind of person. Granted, his traditions are much, much older than Issei's, so anything Issei does is going to come wildly out of nowhere.
He does research, and apart from coming up with fascinating records of marriage and property laws that are nevertheless unhelpful, he does find a few useful pieces.
The thing about ancient times is that there's a lot of time that qualifies as ancient. He's not even sure if this will work or if he should just not risk making a fool of himself...
A man shouldn't be so insecure! He shouldn't fear foolishness when his love is on the line! Is he a worthy husband for Diarmuid or not?!
"What are you thinking?" Diarmuid asks.
Oh. That's right, he's having this debate with himself with Diarmuid sitting next to him on the grass. Before he can stop himself, he asks, "Would you like to hang your washing next to mine?"
...To his ears that sounds incredibly stupid. It must have been from the right period, though, because Diarmuid turns a bright red and ducks his head a little. "Do you mean that?"
"Of course I mean that." Issei places one of his hands over his boyfriend's. "Will you marry me?"
Diarmuid swallows around the lump in his throat. "Yes. Yes, I will."
Then he's immediately tackled in a hug. Well, Issei told himself he wasn't going to fear looking foolish.