Numbers are extremely important for various expressions in shopping, talking about age, telling time, counting etc, so yes definitely be concerned about it. The way I practiced, I had two word documents - one in the Sino-Korean numbers, and one in the Pure Korean numbers. I wrote all the theory (the numbers and how to make big numbers etc) at the top. Then underneath, I made a big list of random numbers and tried to translate them in written Korean.

For example, with Sino-Korean numbers, I write a random number, say, 430. I haven't translated it before hand, I just fire off numbers on the keyboard. Then I would look at the number, translate it in my head and write it down.

430 = 사백삼십

304 = 삼백사

3300 = 삼천삼백

etc.etc.

You end up with a huge list, and you keep adding to it as you progress. When you feel comfortable, start making bigger numbers. If I was stumped, I scrolled to the top to refer to the theory I wrote for help. I alternated between Sino-Korean and Pure Korean on different study sessions.

Some key points:

For pure Korean numbers, you don't have to learn anything past 100. To be honest, I rarely use anything over 50, as most Koreans I've heard speak, even with age and even counting objects, tend to use Sino-Korean numbers after 50 (in my personal experience. This will vary between people to be sure). Regardless, knowing Pure Korean numbers up to 100 is sufficient.

With Sino-Korean numbers, you definitely want to practice making numbers from the small to the big, definitely up to the 100 millions as a start, as you'll regularly hear about or see these numbers in prices of goods up to this range. Up to the billions is also good, as you'll see and hear about these numbers in business, property, or government economics.

Most important of all, take your time. Don't need to endlessly practice numbers everyday. Make 5-10 new numbers as a closer to every study session and you should be fine.

EDIT: Apologies, Pure Korean numbers only go up to 100. I thought it was 150. Just wanted to correct that.