Here's How:

To tell exact time, say a cardinal number such as один, два, три, etc. + час/часа/часов + ровно.

If the cardinal number is 1, use the word час

If the cardinal number is 2, 3 or 4, use the word часа.

If the cardinal number is 5 to 12, use the word часов.

For example, три часа ровно (3:00).

When there remains less than half an hour till the beginning of a certain hour, you say без + the number of minutes remained + минуты/минут + the hour that will come.

If the number of minutes remained is 1 or 21, use the singular genitive form of the numeral, namely одной, двадцать одной. You should add the word минуты to such numbers. Instead of 15 minutes use the word четверти.

If the number of minutes is other than named in step 7, use the plural genitive form of the numeral. You should add the word минут to such numbers.

To name the hour that will come, say the word час for 1 hour and correspondent cardinal numbers alone for others, namely два, три, четыре, etc.

For example, без пяти минут два (1:55); без четверти десять (9:45).

When there passed less than half an hour since the beginning of a new hour, you say the number of minutes passed + минута/минуты/минут + the next hour.

For the number of minutes passed, use a cardinal number in the nominative.

If the number of minutes passed is 1 or 21, use the word минута.

If the number of minutes passed is 2, 3, 4, 22, 23 or 24, use the word минуты

If the number of minutes passed is 5 and more (except 21, 22, 23 and 24), use the word минут. Instead of 30 minutes use the word половина. Instead of 15 minutes use the word четверть.

To name the next hour, say первого for 1 hour, второго for 2 hours, третьего for 3 hours, четвёртого for 4 hours, пятого for 5 hours, etc.

For example, двадцать две минуты девятого (8:22); половина десятого (9:30); четверть десятого (9:15)

Tips:

The stressed vowels are underlined in Russian words.

If you want to designate the time of the day, add at the end of the time phrase the word утра for morning time, дня for noon and day time, вечера for evening time, and ночи for night time. For example, пять минут второго дня (1:05 pm); пять минут второго ночи (1:05 am).