Bought a house, backyard has slight slope towards the house
However, you do not want a "level" yard. You want it to slope away from your home. Make sure that you have at least a quarter of an inch drop in elevation per foot of linear travel away from the house. For example, in ten feet of distance (10 X 1/4 inch = 10 quarters of an inch, or, 2 1/2 inches drop) you will need the earth to slope down and away about 2 1/2 inches. And, therefore, if you go twenty feet away you will need 5 inches of positive drainage slope away from your structure. From there you may flatten out to more level ground but always always always maintain positive drainage away from your home.
what you would do is get two stakes and place them at the two ends of the area you want to make level. String a line tautly between the two stakes and hang your level on it in the middle of the string, then adjust the height of the string until the level indicates the string is, well, level.
After you are certain the string is level and straight--not sagging, measure from the area under the string that is longest and remove the dirt beneath the string until every area matches that measurement. You could also add dirt to the low areas instead of remove dirt from the high areas or do a combination of the two--what is important is that the area beneath the string be the same measurement along the total string length.