IT IS HARDER THAN IT SOUNDS.
There, that's my disclaimer.
The truth is it took a lot of preparation and honestly it would be easier to find an abandoned M7M. But, if you play with some sort of rules or code of conduct, then maybe this will be useful.
SETTING:
Since I didn't have an Elephant yet I chose an Arena as my target. It's fast, which makes it more challenging but the idea is that the exercise should be an analog to boarding an M7M.
The first attempt involved the two TP's and my Hyperion. I only managed to get 5 guys out of 20 on board, and they capped it. Ordering the TP's to board by themselves while you suppress the shields is just untenable. Overall a very frustrating and essentially worthless exercise since it's not analogous to boarding your first M7M unless you already have 20 elite marines, in which case you already have a bunch of boarding experience and don't need to read this post

So I scrapped the whole operation and tried a different approach.
TOOLS:
1 Heavy centaur prototype / (any Ion-d platform will work).
2x TP (I used the Express)
Transporter devices
20 fully trained rookie marines. (average fight skill ~ 30, trainable skills all 100)
M3/M4 fighters armed with PBE's / (I used L's and M's)
TACTICS:
Ships generally follow a route. It's important to figure out where it will be in the future. In the case of an M7M in a pirate sector, figure out which gate/sector it's going to. In the case of a brazen sector theft, figure out it's patrol route. You want your TP's in front of the target and you want them to have transporter devices.
Look around for hostiles. If need be take them out first. When spacewalking you don't need any additional complications.
Make sure you have plenty of fighters with PBE's (~10). I say PBE's because I've found that they work the most effectively for the 'attack shields' wing command. Anything slower or longer range results in hull damage.
Scan the target. See if it's packed with enemy marines/sentry lasers etc. You may have to burn off the offending upgrades with your Ion-d. Hopefully it's defenseless, but sometimes life is complicated.
IMPLEMENTATION:
The Arena had hull polarization. On a TL that's guaranteed spacewalk failure, so it has to go. I also know from the initial attempt that the local Python will get involved, making things messy. It has to go too. Some observation time shows that the Arena is flying a 2 point patrol in a generally N-S orientation.
I had one TP follow the other and sent them to an ambush point. I called in my J with 5 L's and 5 M's, all armed with PBE's. I set the fighter wing up at an interception point a few km from the ambush point on the Arena's path. I set up my HCP armed with 2 Ion-d's and a firestorm with them. Then I called in my Minotaur and capped the Python. (An M2 is a terrible thing to waste

I had to ram a police scorpion or two (to minimize rep loss) and sent the Minotaur and Python out for resupply/refitting. I then flew to the HCP and transferred ships. At this point the sector was pretty much undefended. My Split rep had taken a bit of a beating, but I can deal with that later.
Soon enough the Arena approached the interception point. I smacked it with a firestorm, bringing it's shields down to 0, and proceeded to roast off the components. At 98% hull the polarization software got burned off. I ordered the xenon wing to attack shields and sped off to the TP's.
I entered the lead TP and ordered the other TP to stop. I then flew towards the approaching Arena until the second TP was ~ 4km away. As expected the fighters were maintaining the Arena's shields at ~2-5% and the Arena was heading right for the TP's. When it got in range (just under 2km) I used the additional ship commands to launch all marines, then immediately switched to the second TP and launched all marines from it too. Except it didn't work.

I hadn't targeted the Arena so I had to scroll around to find it, at which point it was passing me, so the marines ended up behind it where they would never catch up. Abort.
On about the seventh try I finally got the timing right. I launched the marines from the second TP just as the marines from the first were making hull contact. They basically got run over by the Arena so they joined the fight just before breaching the hull. Somehow one marine got left behind, and one of the glitchy xenon fighters shot him,

I did lose an M that rammed the Arena but there are plenty more where it came from so oh well, and considering that I have hundreds of elite marines I couldn't be bothered to reload to save a rookie. I guess he was a very expensive fully trained rookie, but to be honest I'm getting a little bored of training up the last 30 or so. It's getting in the way of my xenon accumulation project.


Cleanup took a some time, apologizing to any red ships, re-deploying sats, refueling the J and HCP etc, but I would say the exercise was successful, so from now on I will recommend it to anyone who wants to board capitals the really hard way...

JUST GET AN M7M...