I had to get an Apple Advisor to talk me through it, in my case the source was an external backup drive.
But beware. In my case at least, not everything came over and there were no error messages warning me of that. Some gaps were obvious, e.g., the Mail mailbox tree structure came over but none of the messages. It took a Senior Advisor to fix that and it took him awhile. Software that is not compatible with Yosemite did not migrate but that is to be expected I guess. But here is the killer. Yesterday afternoon I was looking for a pdf file and could not locate it anywhere. Fortunately I still had the old iMac and found it there.
I then compared the folder in the two iMacs and found that a couple of pdf files had not been transferred. Important files. That scared me so I spent the next few hours going folder by folder through Documents. In many cases the files were there but with a slightly different size, e.g., 130k on the new and 135k on the old. But they seem to be ok otherwise.
BUT, there were four complete folders that did not make it over plus about 15 other files. Very scary and I hope I didn't miss anything.
If there is a pattern I cannot figure it out except that the file types are all either pdf, Excel or Word. Some are old and some fairly recent. But all created before Migration was used. But many hundreds of similar type files came over fine.
I have no advice except to be careful and check that your important stuff makes it over.