In RMSS/FRP, you have two sets of Stats:
Temporary stats represent what your abilities currently are, while Potential stats represent the highest you'll ever get. For pretty much everything, you'll use your temporary stats. The Potential stats are there to limit how high you can go through stat gains. MERP does not use temporary/potential stats.
That said, the process is pretty simple to use, but to do so would require you to add potential stats to your MERP characters.
When you do a stat gain, you'll roll two d10 dice. This is not a d100 roll. If you roll two 1's, 2's, 3's, 4's, or 5's, ( for example a 1 and a 1, or a 2 and a 2), then you reduce the temporary stat by that number. If you roll two 6's, 7's, 8's, 9's or 10's, then the stat increases by the total of the dice. For example, if you roll an 8 and an 8, you increase the temporary stat by 16.
Otherwise, you have to compare the temp stat with the potential. If the difference between the two is 20 or greater, then you add the total of the dice to your temporary stat. (So a 6 and a 9 increases the stat by 15.) If the difference between the two is less than 10, you add the lower of the two dice, and if the difference is between 10 and 20, you add the higher of the dice.
You cannot increase the temporary stat past its associated potential stat. So if you have a potential of, say 87, then you'll never raise the temporary stat above 87.
In RMFRP, to determine your temporary stats, you get 660 points to divide among them. For potentials, you'll roll dice. It's summarised on a table, but what page it's on in the RMFRP book escapes me. In RMSS, it's on page 54.