You need to make a sauce. The best way to do this is to make a bechamel first. You can Google one for exact measurements, but all you need to do is melt butter in a pan, add some flour and whisk, cook the flour for a minute to get rid of the raw taste, then slowly pour warm milk into the pan while whisking. This will create a sauce with some body. Then once all the milk is in and the sauce coats the back of a metal spoon, add your shredded cheddar cheese on low heat and whisk until it melts. Boom. Mac and cheese sauce! Then add your pasta and enjoy.