Tournament Round robin to bracket

An example tournament has two stages, groups play + bracket play. When the groups stage ends, the x top of each group go to bracket stage. In the brackets stage there are two basics rules:

a winner of a group should not be matched against a winner of another group (only against 2nd or 3rd place)
no two teams that played in the same group should be matched against each other.

For example, if we have 6 groups and top 2 each group advance to bracket:

If i put the teams i get the matches, but i dont know how get the correct order for put then in the bracket.

1A 1B 1C 1D 1E 1F 2A 2B 2C 2D 2E 2F BYE BYE BYE BYE

1A vs BYE

1B vs BYE

1C vs BYE

1D vs BYE

1E vs 2F

1F vs 2E

2A vs 2D

2B vs 2C

Attachment 36017

Another example with 4 groups of 8 teams and all teams advance to bracket:

I apply the snake pattern:

1A 1B 1C 1D 2D 2C 2B 2A 3A 3B 3C 3D 4D 4C 4B 4A 5D 5C 5B 5A 6A 6B 6C 6D 7D 7C 7B 7A 8A 8B 8C 8D

Matches:

1A vs 8D

1B vs 8C

1C vs 8B
...

Attachment 36018

Any ideas how put matches in correct orden in the bracket?

Thanks.