Thank you all for understanding and accepting my halt 

Anyway 

DarkJack in answer to your questions 
How does one actually go about moving from one realm to another, and what exactly is it that divides them? Is there a special kind of magic or some hidden portals or something that connect them? 
There exists entrances/ portals throughout each realm that will transport a person 
As well as rituals and spells could get a person there
Eg. A certain symbole drawn on a wall or in the ground combined with certain words could possibly transport you
The entrances within the mortal realm would be well hidden and quite possibly camouflaged to look mortal. As well as guarded so no mere mortal wonders through. 
Limblel and Urbaniel is the one exception to this though as Urbaniel and Limblel has a direct connection and is the only case where one can trek from the grounds of one to the other. However to exit either, one is in need of a portal. The connection is a bid to halt any whom escape Urbaniel for they can face there fate in Urbaniel or disappear and get lost in Limblel. Finding a portal to exist Limblel is extremely difficult and creatures guard these doors.

Is there a price to pay for crossing over?
There is no price to pay for crossing over 

That reminds me, how does using their powers affect beings in this world? Does it tire them, drain them somehow, or are they capable of invoking their various powers indefinitely without suffering any detrimental effects from it?
It would depend on the power, the species, and the person 
I believe most would be unaffected however spontaneous reactions may occur
Also whilst one dwells within a certain realm for longer they may find that they build immunity to any change that stems from dwelling in that realm 

Prince
Sorry for the delay 
But even your second draft has no faults 
Your accepted and I, and others be keen for you to start

Working on my post :)
May I do a roll call?