It's not difficult, but if you're not familiar with routers, watch some general woodworking videos on how to use them, practice on some scrap wood, go slowly taking off small amounts per pass, follow the direction of the router bit and use a template. Simmnet in the UK make nice ones.
If you're unsure what you're doing, it's really easy to <expletive> up the instrument. Be careful.
I've never used a Dremel for this, but a small flush trim style router would be better as it has some power, but no so much it'll be like a rodeo bronco if it bites into something in the wood like a denser grained section.