As it is already explained in the chapter of verb that modal operators are those auxiliaries which have only one or two tensed forms. They are also called modal auxiliaries or modals in short, they are;

can, could, may, might, ought to, shall, will, would, must, should.

If any sentence has modal operator as mentioned above you have to convert it into passive voice by following the simple instructions below.

Basic changes

Subject converts into object.
Object converts into subject.
be is used with modal operator.
Present Participle form of the verb is used.

Use of auxiliary

As for auxiliary the same modal operators are used followed by be.

Examples

I couldn’t understand you.
You couldn’t be understood by me.

We can make mobile software.
Mobile software can be made by us.

You may operate this machine.
This machine may be operated by you.

He might tell his secret.
His secret might be told by him.

You ought to practice law.
Law ought to be practiced by you.

I shall learn English.
English shall be learnt by me.

He will break rules.
Rule will be broken by him.

I would read newspaper every morning.
Newspaper would be read by me every morning. OR, Every morning newspaper would be read by me.

You must follow these instructions.
These instructions must be followed by you.

You should read more books.
More books should be read by you