If you buy a pot of menthol crystals from boots, you can either dilute 1mg of crstals in 10ml of PG and add it drop by drop to your juice, or you can add the crystals direct into the juice.
If you add direct to the juice start very very small by crushing a single crystal and add a tiny bit of the powder at a time, make sure it is fully dissolved into the juice before using it; otherwise you may think it's too weak, add more, and then find that when it fully dissolves it's way too strong.
Either way, to speed up the diluting process you can put your bottle of juice/PG into a warm water bath to soak and give it a vigorous shake every 5-10 mins. Some people microwave the PG+crystals for 10 seconds and shake then repeat until dissolved. Not sure how a pre flavored nic juice would react to being microwaved though, so would avoid that route.
I made some concentrate in a 5ml bottle. Half fill with menthol crystals and top up with pg, then zap in microwave for 2 seconds max found this out when left on for about 8 seconds. Bottle went soft and deformed. Used it straight away in a tobacco mix at 6% not too overpowering but the crystals had been kicking around a while. Good luck