Just to add to what Meshico said, make sure to look at the stat numbers before melding. If you are adding stats to gear and won't get the full value listed on the materia, the numbers will be red and it will tell you how much the meld will give you. In some cases you will want to use lower tier materia instead as it would be more cost-effective.

Don't be that dope that has 3 +0 Strength melds!