RAID0:
Given 4 slices, each ~5g:

First, need a metadb, I created a 100MB slice on c1t1d0s0 (which I am NOT using for the RAID, entirely separate drive) and ran this command to initiate the database. It is a good idea to mirror the database in a minimum of 3 positions, but that is beyond the scope of this tutorial
metadb -a -f c1t1d0s0

Then, it’s as easy as 1 command to bring multiple drives into one slice/partition with the following command:
metainit d100 1 4 c2t2d0s0 c2t3d0s0 c2t4d0s0 c2t5d0s0
NOTE: I already created slice 0 on each of the drives.

To see the status of your meta-slice:
metastat d100
d100: Concat/Stripe
Size: 40878080 blocks (19 GB)
Stripe 0: (interlace: 32 blocks)
Device Start Block Dbase Reloc
c2t2d0s0 0 No Yes
c2t3d0s0 4096 No Yes
c2t4d0s0 4096 No Yes
c2t5d0s0 4096 No Yes

Device Relocation Information:
Device Reloc Device ID
c2t2d0 Yes id1,sd@n6006048cb0ca0ceeef67fa7a33ce4c94
c2t3d0 Yes id1,sd@n6006048cb275dda20f654d7248d17197
c2t4d0 Yes id1,sd@n6006048c5aa658e3c69370f2bad75bc0
c2t5d0 Yes id1,sd@n6006048cc092136a695a21eeaa948f88

See? Now we’ve got a 19GB slice. Feel free to newfs /dev/md/dsk/d100 and mount it somewhere fun.

Next up: RAID1
This is actually not as hard as it looks. First, make sure you init your database like the first step from above. Then initialize your first meta slice:
metainit d101 1 1 c2t2d0s0

Then, create the mirror for that slice which will become your final RAID1 slice by issuing the following command:
metainit d100 -m d101

Then initialize the other slices in your mirror, in this care there are 3 additional slices:
metainit d102 1 1 c2t3d0s0
metainit d103 1 1 c2t4d0s0
metainit d104 1 1 c2t5d0s0

From there, it’s quite easy to finish it up by attaching the mirrors:
metattach d100 d102
metattach d100 d103
metattach d100 d104

Then, monitor metastat for the sync progress percentage until all the mirrors are sync’d. Finished!
metastat d100