Expressing "to be sure to" with 万万 and 务必
万万 and 务必 are modal adverbs (语气副词), they expresses subjective judgment or assurance toward an action.
The adverb 务必
The adverb 务必 
is used to emphasize that an action must be performed.
In a sentence, it is placed before a predicate that is expressed by a verb.
is used to emphasize that an action must be performed.
In a sentence, it is placed before a predicate that is expressed by a verb.Examples
你务必要按时到达那里
You must get there on time
nǐ wùbì yào ànshí dàodá nàlǐ
明天的会议请大家务必出席
Please be sure to attend tomorrow's meeting
míngtiān de huìyì qǐng dàjiā wùbì chūxí
座位有限，务必早日预订
The number of seats is limited, so be sure to book them in advance
zuòwèi yǒuxiàn, wùbì zǎorì yùdìng
The adverb 万万
The adverb 万万 is mostly used in negative sentences.
It is usually followed by the negative adverb 没 or 不 .
This combination has the meaning "under no consideration" and emphasizes that an action doesn't need to be performed.
is mostly used in negative sentences.
It is usually followed by the negative adverb 没 or 不 .
This combination has the meaning "under no consideration" and emphasizes that an action doesn't need to be performed.Examples
万万不能开着煤气出门
When you leave your house, never leave the gas on
wànwàn bù néng kāizhe méiqì chūmén
他万万没有想到他会成功
He never thought he would succeed
tā wànwàn méi yǒu xiǎngdào tā huì chénggōng
我老板给我提供了我万万梦想不到的高薪
My boss offered me a salary that I would’ve never dreamed of
wǒ lǎobǎn gěi wǒ tígōng le wǒ wànwàn mèng xiǎngbudào de gāoxīn