If you have a ship that can visit outstations (rather than the larger coriolis-type space ports), pick a small cluster of Empire aligned systems on the fringes of inhabited space. An example is in the location of Dharwar, or possibly Aling, but it doesn't have to be these. They will offer a lot of courier missions to the same outpost, which you can stack up to 20, and offer ++ Rep to boost your progress each time. Keep going round a group of, say 3 to 5 systems, and you will accrue a lot of progress in a short time.
I found it got boring after a while, so I went off and did something else, such as participate in a CG, but will have to do more of these to continue my progress (grind) through to Duke in Empire.
If you think these courier missions don't pay very much, you are right, but as your rep increases they will offer you other missions, including planetary scan missions, which pay a lot better. You can also boost your income by running some passenger missions too. It doesn't have to be all courier missions, but they seem to offer the best Rep ++.
Look out for inexpensive (less than half a million) charity donations offering Rep +++, which come in handy for progress without breaking the bank.
Also, this thread is helpful for understanding the stats:
https://forums.frontier.co.uk/showt...-some-data-amp-donation-mission-stats-for-you
