Sure! Here are the completions for your sentences:

1. I must reject this plan.
I will **not be going along with it**.

2. It didn’t escape his attention that she was upset.
He couldn’t **help but notice her distress**.

3. There’s nothing more to be done about this matter.
Nothing more **can be done**.

4. A new car is twice as expensive as a second-hand one.
A new car is twice **the price of a second-hand one**.

5. I don’t seem to be able to understand this instruction booklet. (HEAD)
**I can’t get my head around this instruction booklet.**

6. David doesn’t seem to be able to get on top of the new computer system. (GRIPS)
**David hasn’t got a grip on the new computer system.**

7. Angela is just recovering from a nasty bout of flu. (OVER)
**Angela is just getting over a nasty bout of flu.**

8. Maggie’s little boy is just learning to put on his own clothes. (DRESSED)
**Maggie’s little boy is just learning to get dressed by himself.**

9. That little joke really caused a lot of trouble for me. (WATER)
**That little joke really got me into hot water.**

10. Molly finds it difficult to accept the idea that her daughter is grown up. (USED)
**Molly isn’t used to the idea that her daughter is grown up.**

Feel free to ask if you need further assistance!