Personally not knowing the strength of the coffee extract, find your sweet spot for that flavor alone first.
Use a dropper and a 3ml bottle add 100 drops in combination of vg/pg mix to coffee extract (1 drop = 1%).
Honestly I'd start at 1% coffee and 99% mix. Bump up the coffee until you hit your sweet spot, maybe even a little past that.
Then add cream, hazelnut, etc to taste.
I find if you use Flavor Arts flavorings like Hazelnut and Irish Cream like
@sparkky1 suggested above start at a very low percentage (1% to 2%) as they can quickly overpower a mix.
I haven't used any TFA flavors so I am no help there.
I haven't been in the DIY game long, I am sure some of the more seasoned users can help out more.