How it is possible to opening upgrade in MDI

Question

I have a question similar to one already asked. I have an MDI app, and I would like people to be able to open forms from another app I have created as MDI children in the MDI app. The answer to the question similar to mine said something about creating a dll, but I didn't really understand it I guess. Could somebody please tell me if what I'm trying to do is possible, and if so, point me in the right direction? Thank you so very much.