Once you are confident with singing a scale in tune try larger intervals.
Larger intervals can be more difficult to hear and execute. The distance between Do-Mi is a major third, which is equal to 3 half steps. The distance between Mi-Sol is a minor third (slightly smaller), which is equal to 2 half steps.
A great way to get this pattern in your ear is to sing it in all twelve keys. Here is the pattern written out in the key of C.