You may think it is as simple as just asking as a rather general, "how can I add grains?" but honestly, the right answer is a very big "it depends". However, some initial things to consider.
Not all grains, nor all grinds of grains absorb the same amount of moisture, nor do they all absorb it in the same amount of time or in the same way. For example, if you have a cup of of cake flour and a cup of whole wheat flour, and add the same amount of water to both, one would end up a dryer feeling dough than the other because they absorb water differently. This same idea extends to all the additions you might be making. If you are adding steel cut oats vs. light rye flour vs. pumpernickel flour vs. spelt vs. etc... you will need to adjust the amount of water. And the truth is, this is likely best learned through experimenting with the grains you have available to work with (due to things like storage location, humidity of the kitchen, etc... these adjustments won't always be the same for everyone, so buy a few small packages of other grains and experiment). You already have an idea of what texture of dough you're comfortable with though, so your starting point is likely to mix your ingredients together, leaving the flour and grains as the last addition and adding as much as needed to get the desired texture of the dough. Don't let the measuring cups dictate what bread you have, let the dough dictate how much you measure.
Second, as mentioned above, some grains absorb water slower than others. So, for example, if you're using whole wheat flour instead of a general all purpose white flour, it may help if you mix up all of the ingredients and then wait 10 or so minutes before kneading so you can see if its the right consistency/texture you want and adjust from there before kneading.
Additionally, some grains take so long it is better to let them fully hydrate (and potentially soften some) before mixing them to the dough at all. This is easily accomplished with what is often called a soaker, which is basically just mixing any grains with water (generally just enough water to barely cover the grains) and letting it sit for several hours to overnight. Then you'd add the soaker in as you make the dough, making sure to allow for the additional water you added via the soaker. Another option would be to cook the grain such as in the case of recipes that call for leftover oatmeal or cooked rice to be added to the dough during the initial mixing of the dough. The important thing to remember is simply, if you add a big hard seed to the dough, it will take the same things to soften the seed in a bread to an enjoyable point as it would in any other circumstance: moisture for an extended period or hot moisture for a shorter period. You'd never take raw steel cut oats, pour them in a bowl, add some cold tap water and stick a spoon it it and eat... you're going to have to soften them at some point to make them enjoyable via time or temperature.
I wish I could give you more specifics, but it depends so much on what specific grains you are wanting to add and what end result you are aiming for (ex: if you want to add more crunch or grit to the texture of your bread to make it feel heartier you'd treat it differently than if you're simply trying to hide whole grains in the loaf you're already used to).
My best suggestion would be to start with some trusted recipes for whole grain breads to get a feel for how the grains work, then you can adjust your own bread recipe and process to include grains once you know them better. If you need some sources, there are some great resources for bread recipes, including whole grains, including "The Laurel's Kitchen Bread Book: A Guide to Whole-Grain Breadmaking", http://www.thefreshloaf.com/ and http://www.kingarthurflour.com/recipes/whole-wheat/whole-grain-