This is not too hard. Depending on what you want to do, the details will change.
FIRST question: are the drives now mounted in the machine they were originally used in? Can they be mounted there, even temporarily? Doing that makes it easy for starting the "un-RAIDing" of the disks, and it is VERY important if you need to preserve any of their data.
IF you can reconnect them into their original RAID1 configuration then you may be able to copy off any data you want to save. IF you do this, be sure to CHECK THE COPIES and make sure you got everything you want and they all are readable. THEN you can proceed to clean off the disks.
Even if you are not saving old data, the easiest first step may be to mount the drives back as if they were in the RAID 1 array in the original machine. Then you use the RAID management tools in that machine to BREAK the RAID1 array into two separate non-RAID disks. This will give you two disks full of their original data and fully usable as stand-alone units. This step is basically the same one you would have used to start repairing a damaged RAID1 array by breaking it to replace a faulty HDD - only this time you don't have the repairs to do.
Now you are dealing simply with plain old used hard drives (two of them) that happen to have old data on them you might want to get rid of so you can start fresh. Even if you can't do all this so far - that is, if you cannot re-mount them in the old computer RAID system, etc. - you can start from here, anyway.
The best way to set up a used disk to re-use is to get rid of all its old Partitions and then establish new ones. The tools for this are built into Windows in the Disk Management utility. Mount each disk in turn (or both at once, if you choose) in a computer and connect up. Start Windows. Click on Start at lower left and then RIGHT-click on My Computer and choose Manage from the mini-menu. In the new window expand Storage on the left if necessary and click on Disk Management. The right part of the window will become two panes. Concentrate on the LOWER RIGHT pane. It scrolls so you can see all the hardware drive units attached. Be VERY CAREFUL here because the next steps are going to DESTROY all data on the drive you select, so do NOT choose the wrong one!
Each hardware unit is represented by a large horizontal block. Each block has a small label block at its left end. To the right of that is one or more large sub-blocks, each representing one Partition on this drive unit. There may be only one; there may be more than one; there may be one main Partition plus some other area called "Unallocated Space". Working ONLY on the drive you want to empty off, RIGHT-click on each Partition it holds and Delete this Partition. When the unit is ALL Unallocated Space, RIGHT-click on it and choose to Create a Primary Partition. You can choose how large it will be, up to the full disk size. Assuming you do not want it to be a bootable drive (you're only going to use it for data), do not make it bootable. You MAY be in a wizard by now that also lets you specify options for Formatting the drive with a File System. If so, choose the NTFS system. I recommend you choose to do a Full Format. This will do exhaustive testing of your drive for any errors, but it will take HOURS to process one drive, so be patient and do something else. With the options all set, run the task. On the other hand, if those Format options are not apparent, just run the Partition creation task first. When it is done, RIGHT-click on the new Partition and choose to Format it, then set the options as above and run that task separately. When it is finished, exit out of Disk Management and reboot the machine to let Windows know about the new disk you have.
Repeat all this for the other disk. Then shut down and remove the two disks, if necessary - they should be completely ready to use in any Windows machine.
Now, IF you were planning to use these disks as boot drives in some other machine, you can stop after Deleting all the Partitions on each drive. The Windows Install process does a very good job of Creating and Formatting for you as the first part of its work, so don't bother doing that yourself if not necessary.
RAID1 back to normal? you must go into your raid controller (depending on what your using) and disable the raid mode or delete the raid array etc - you need to know how to enter the tool all depending on card/controller/motherboard etc
another way is to remove one hdd from the array physically, then zero write it with whatever tool you can get your hands on, then replace it with the other and zero write it too then plug both back in but you shouldnt have to do that.