Merging Two Objects Without Seams.

I have a pretty elementary knowledge of Blender, so I assume (and hope) this will be a simple fix for someone who knows what they’re doing. How do I go about attaching one object to another and ensuring there’s no visible seam. I’ve got an example with a house I built and a stained glass window, it’s the exact right size, but still is very difficult to manually fit in there without leaving a visible seam.
Here’s an image of the two pieces that may help with visualization.

I found that If I join the two objects then I could get rid of most of the seams by manually merging lots of vertexes, but it was a very time consuming process and leaves the whole thing looking very messy. Is there a way to do it with edges or faces that I should know about? 

Here’s about what I’m looking at if I join the two and go into edit mode.

My gratitude to any who read and/or offer help.

It really depends on what you’re going to do with the model. There are many ways to go about this
problem a little confusing though…
Without a blend file, I don’t think I can help nearly as much, but I’ll try

One way is to scale the window up slightly more than the frame, select the window first, then the frame and join
You can also use the “Boolean” modifier, set it to the appropriate setting, probably difference, and apply to the window

In the third picture, I understand that you’re trying to join those two objects?
In that case, you can select multiple vertices and join them as one face

But I’d say half of the time you honestly don’t even have to do that

Hope this helps :slight_smile:

It may not help you now, but in the future consider starting your window from duplicated faces from the main mesh. In the main mesh, select the window opening faces, duplicate them, and separate them with the ‘P’ key. Now you have a perfectly fitting window frame.