LITTER BOX TRAIN YOUR RATS
Rats are, for the most part, very clean animals and will litter box train in most cases.  The reason I say most cases, instead of all, is that there are always acceptions to the rule.  All rats have an individual personality and some are more stubborn than others.  With some patience on your part, it can be accomplished and is well worth it with making cage cleaning easier on you.
Many different things can be used as a litter box for your rats.  You can buy a  corner litter pan (ferret supplies), a small cat litter box, storage container bottoms, some use cardboard shoe boxes and throw all out on cleaning day.... well  you get the picture here.

Make sure to use plain cat litter. Do not use scented or clumping litter, since they may at times nibble on the litter. The scented an clumping types of litter can make them ill. Also try to get litter with a label stating it has Low Dust, some brands have a vertual Cloud of dust that comes up when pouring it into the litter box. Lots of dust particles can irritate your rat sinuses.

Start young if possible.  Babies will learn to use a box pretty fast.  They will learn even faster if their mother is also litter box trained.

Watch to see where your rats tend to relieve themselves. Often they will use the same corner or spot over and over. Place the box with litter in their cage in the spot you have seen them using already.  Take a tissue and pick up droppings and place in the litter box so they smell it in the box.  Sometimes that is all it takes. 

If they are a little more difficult, watch them and when they start to relieve themselves pick them up and place in the litterbox.  This is sometimes easier (for us, not the ratties) done in a play area where they are not in the confines of a cage. Once you have them using the box in a play area, a box can be placed in the cage with them as well.

If starting with older rats, try placing the litter box with plain litter in their cage without instructing them first.  Many, if given a place to relieve themselves, will use a box without any coaxing.  Since rats like to be clean, you may find they may train themselves. If not then try training them as you would a very young rattie.

When they use to box properly always make sure to praise, pet or give a treat to let them know they are doing right. The experience Can be tedious but can also be a great bonding time and even sometimes a little fun too.
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Above you will see some photos of a couple different litters of rat babies as they went through their Litter Box Training.
This was my favorite picture of rat babies litter box training. It was taken years ago when I used to raise these little ones. This litter decided it was a Game. I litter box trained them in their play area ( a mid sized plastic swimming pool). To train them I would Praise them with GOOD JOB every time one used to box properly. After they had all been trained anytime they were in the play area (shown equiped with a litte box), they would relieve themselves in the box, BUT they would also ALL run and jump in the box anytime they saw me come in the room, to check on them, in order to get a 'GOOD JOB" and a Treat.   :)