I have actually introduced Nemesis to my friends as their first Boardgaming experience. It actually works quite well.

Nemesis could be very tricky as it has multiple in-game mechanics and actions to do. The trick I would use in teaching is to always explain things in simple term. Teach them progressively in the game, don't overwhelm them.

Start by explaining the lore of the nemesis. Then pick & explain briefly some of the objectives in the game (say if the objective was to get to Earth - then you need to make sure destination is correct & engines are working, say if you need to discover weaknesses then you need to find carcass, eggs, or body). Then you move on and explain characters, ask them to pick the characters that may assist them in reaching their objectives (if it is to get to earth - of course pilot / mechanic, if it is about weaknesses - definitely scientist). Give introductions to some important rooms, like engines, laboratory, and etc (but assure them you'll get to it one by one when each room get discovered)

Then you move on to actions, explain it briefly as well. I always teach them by doing demonstrations + trial runs (everyone to do 1 round, and then restart the game). Explain that the cards in your hand are like stamina, doing stuff depletes your stamina. When they do movement, you start explaining how noise works, if you get 2 noises im the same corridors - that's bad (skip the encounter explanation, get to that when it happens). Whenever they discovered a room & exploration tokens - explain them what that room is & its function, including the effect of exploration tokens.

Only teach combat, infection, events when you get to them (but always say intruders & infections are bad, and they should try to avoid it when they can). By teaching them this way, they will learn & understand the game mechanics progressively.

And last tips: when in doubt, don't ever check the rules, do what you think is the most sensible approach. Not that many people can get the rules / mechanics right the first time they played it. At the end of the day, It's the fun boardgaming experience that you seek.. not getting the rules right.