Keep in mind that the gem comination in the picture is just to show what it looks like, the stats on the bow is not the result of the combination of these two gems.

The amount of gems in this picture is 50 in each slot because the level of the item being blessed is 50. If you have an item that is level 34, the amount of gems in each slot would be 34.

You can bless an item as many times as you want but the next time you bless an already blessed item it will be overriden. So make sure you bless the item with the combinations you want.

To maximize a stat on an item you would need to combine two gems in all the slots except the lonely slot.

There is one slot that cannot be combined, this slot gives a raw stat. If you for an example put a red gem inside it will give you damage. You can put any gem inside this slot for different raw stats.

Blessing interface looks different for each class and have different slots.

You need to unlock the "Bless Item" feature before you can do this. Just play through the campaign and you will eventually unlock this.
A link gem (orange small triangle) needs to be put in between the gems that will be combined.

Blue = Armour
Green = Max HP
Red = Damage
Yellow = Crit Dmg %

Combos

Red + Yellow = Dmg / Crit Dmg / Crit %
Red + Blue = Armour / AP/ Dmg
Red + Green = Dmg / Max HP

Yellow + Red = As above
Yellow + Blue = Armour/ Crit Dmg / Max Energy
Yellow + Green = Crit Dmg / DoT / Max HP

Blue + Red = As above
Blue + Yellow = As above
Blue + Green = Armour/ Max HP/ Counter Attack Dmg

I hope this helps, i had to experiment around.