All you need to do is create a directory for the models you want to use under the DeepStack directory on C:\DeepStack. Just place the model files in that directory. In
Blue Iris on the AI config tab, make the entry for that directory in the "custom models" slot on the AI config tab.
In the cameras you want to use the models with you need to tell them what models to use. This is under "Trigger" > "Artificial Intelligence". There is a slot for that. You can include whatever objects you want to detect as well. You can also select not to use the default model. That is accomplished by entering "objects:0".
Be aware that DeepStack is case sensitive and every entry has to be made in the same letter cases that each model may use. Some model may start an object with a capital letter, but some do not. Models are separated by commas with NO SPACES in the custom models slot.
This example is a little messed up because I haven't deleted the objects from the stock model, "objects", but does illustrate the upper and lower case use and the comma separations.