First you have to determine what video/audio codecs, resolutions and bitrate are used in the existing file. Use Mediainfo to determine these things. A quick and dirty list of specifications can be found at Wikipedia under Blu Ray.
If the existing video and audio are already compliant to Blu ray specifications you can simply use tsMuxeR to make a new Blu ray structure with them, then burn this output to a disc using ImgBurn.
So that is not difficult in the slightest. You will not get a menu this way, but the output will play in any Blu ray player.
When you start with non-compliant video or audio, you need to re-encode. I generally use MeGUI to make compliant Blu ray video
Here are the settings in MeGUI control panel for the X264 encoder with my preset Blu ray configuration:
There are other sections of MeGUI which control resolution. I’ve been making 720p normally, but its not difficult to adjust to 1080p.
I generally do the audio separately, and normally encode to AC3 448kbps if it happens to be out of spec.