Lets start with a definition of the MBR :
A Master Boot Record (MBR) is a special type of boot sector at the very beginning of partitioned computer
mass storage devices. The MBR holds the information on how the logical partitions, containing file
systems, are organized on that medium.
Besides that, the MBR contains executable code to function as an operating system-independent chain boot loader in conjunction with each partition’s Volume Boot Record.
To make simpler, if a disk is a book, the mbr is the index table to quickly jump to each partitions.
MBR and GPT are just 2 different ways of writing that index (GPT handles a higher number of partitions and also bigger partitions).
So how to convert a GPT disk to MBR?
1.Write down the offset and length of your partition(s)
2.Delete the disk layout (this will not delete any datas, « only » the partition table and boot loader if any)
3.Create Disk (MBR style)
4.Create partition(s) with correct offset (most important) and length. Note that you can always correct these later by editing the MBR (advanced tools).
Choose IFS for a NTFS filesystem, or FAT32.
You end up with a MBR disk with a partition table similar to the GPT you previously had.
Note that you may want to inject a boot loader (nt6, g4d, etc) if this disk is a boot system one.
Also, this operation works the other way : MBR to GPT.