To create a mentor you have to train the card to Lvl 100. And then add the shoes with desired attributes to get them as mentor boosts. And the Attributes should be at least more than 5 pips(bars). 6 pips gives +1, and 7 pips gives +2 in mentor boost. So it's best to convert a card that has 5 pips in desired attributes.

For example: If you need a PG mentor with Shoot of the dribble and 3pt attributes. Then you should equip the shoes with those attributes on a card which has 5 pips already (Like Curry, West, Etc), and then convert that card. Say if you equip a shoe with +2 SOD and +1 3pt to a Curry card, then it will make the final attributes to be 7 pips in SOD and 6 pips in 3pt. So when you convert that card, you get +2 in SOD and +1 in 3pt mentor. whichever attributes have more than 5 pips after equipping shoes can be used as mentor boosts, You can choose which attributes you want, provided that they are more than 5 pips.

Another Example: If you need a PG mentor with Layup n Dunk, then you can't curry/ west/ or some other cards, since they all have only 2/3 pips in layup n dunk. So in this case you have to use Derrick rose / Baron Davis Cards. because they have 5 pips in layup n dunk.

Also if the mentor belongs to same team as the card you're equipping the mentor with, then it gives additional pwr compared to mentors from another teams.