I have a sheet of small riddles that are combinations of letters and numbers that represent by their arrangement common phrases or sayings. The object is to decipher each one.

Post a few... and I don't quite understand the rules?

Yes, please post some. It sounds like a simple substitution puzzle. Like 1=A, 2=B, 3=C, etc. Then you have something like... 312 which means CAB.

You mean the ones such as
There are 24H in a D
There are 24 hours in a day
Or
1000MM are in a(n) M
1000 millimeters are in a meter

High chair