"Lower your shields and surrender your ships."
We can't use {nitmau}, because that means "x1 is lower than x2...", and the English "lower" is used in this case to mean "Cease providing power to...". So, as in most translations, we have to translate the /meaning/, not the words. From the meaning, we need {sisti} "x1 ceases x2", {sabji} "x1 provides x2 to x3", and {nejni} "x1 is energy...".
Now we need to translate "shield". {bandu} doesn't work, because the x1 and x3 of "x1 protects x2 from x3" are events, and shields are an object. So we're going to on a completely different track. The shields are a protective shell made of energy, so we'll use {nejni} and {calku} "x1 is a shell around x2 made of x3".
On to "surrender you ships". We have {randa} "x1 yields to x2...", but that's not precisely correct. The Borg are saying to surrender /the ships/, which means they are saying to give the ships to the Borg. Which means {dunda} "x1 gives x2 to x3" needs to be in there as well. So we'll make up the lujvo {radydu'a} "d1=r1 surrenders d2 to d3=r2..." and use that. For ship, we have {bloti} "x1 is a vessel...", but that word is understood as referring to seafaring vehicles, and we're talking about space-faring ones. Lojban "space" is {kensa} "x1 is outer space near celestial body x2", so again we'll make a new lujvo {keslo'i} "x1 is a space vessel...".
Now that we've got all the concepts translated, it's time to put them together. In the English the Borg are ordering two things, using "and", we can do the same thing with {gi'e}, or {ge...gi}:
{ko sisti lonu sabji lo nejni lo calku be fi lo nejni kei gi'e radydu'a lo do keslo'i}
That's a bit long, so let's use a neat trick to shorten it up a bit. Typically, a lujvo such as {rodbo'e} either means {... brode lo broda ...} or {... brode je broda ...}. We can use that to our advantage, and turn {... sabji lo nejni ...} into {... nensabji ...}, and {calku fi lo nejini} -> {nejni je calku} -> {nencalku}, resulting in:
{ko sisti lonu nensabji lo nencalku kei gi'e radydu'a lo do keslo'i}
Okay, so now we have the first two sentences translated:
"We are the Borg. Lower your shields and surrender your ships."
{mi'a selcme zo.borg. ko sisti lonu nensabji lo nencalku kei gi'e radydu'a lo do keslo'i}
I'll leave it at this for now, and tackle the next sentence in another email.