The first thing you will need to do is get either an Archer/Viking for Cook, Gunner/Swordsman for Home Deco Crafter, to level 80 and do all the merc upgrade to this point. You will need to then remove all the gear on the merc you are converting and visit Alfredo NPC which is located in My Home area in Rome which will bring up this screen;

Click on Gyebaek or Robin Hood then click on employ which will bring up the convert screen like so.

Will cost 10m gold to hire each which is a small fee compared to what they can do and save you gold in the long run. Once you have done this step to both and hired them enter your home and click on the Blue round Home Icon top right corner.

From here to get them into your home click on each one and hit the Activate button at the bottom. Now that we have them placed in our home its time to train them now visit each one and click on them and this menu will pop up.

This window will look the same on both, once here click on the 1st option which is Train and the following window will pop up. and use the following skills like me and click on Confirm.