The usual way to make sure the reader knows they are reading a dream sequence is to make the physical look of the passage different from what goes before and afterwards.

This can be done by writing the dream sequence in italics or by marking the beginning and end in some way, such as whatever character you use to denote the end of a scene or chapter. (Not all authors do this. I don’t. But if you do, then it’ll work just fine.)

Some authors don’t denote any difference in the appearance of the dream sequence and simply depend on their skill as a writer to make sure the reader knows what’s going on. Sometimes this works. Sometimes it doesn’t.

Whatever you choose, good luck. It can be done.