Not very hard. Start with the grid in the upper left corner. Assign the numbers to this grid (it doesn't matter in which order). Now copy this first grid all over, except that for each step sideways you rotate a row in the grid (for instance, put the top row at the bottom), and for each step downwards, you rotate a column (for instance, rotate the left most column to the right). Here's a program:

#!/usr/bin/perl

use strict;
use warnings;
no warnings qw /syntax/;

my $N  = 6;
my $SN = $N * $N;

my @chars = (0 .. 9, 'A' .. 'Z', 'a' .. 'z', '/', '+');

my $mask;

foreach my $x (0 .. $N - 1) {
    foreach my $y (0 .. $N - 1) {
        $mask -> [$x] [$y] = $x * $N + $y;
    }
}

my $grid;
foreach my $X (0 .. $N - 1) {
    foreach my $Y (0 .. $N - 1) {
        my $sx = $X * $N;
        my $sy = $Y * $N;
        foreach my $x ($sx .. $sx + $N - 1) {
            foreach my $y ($sy .. $sy + $N - 1) {
                $grid -> [$x] [$y] = $mask -> [($x + $Y) % $N] [($y + 
+$X) % $N];
            }
        }
    }
}

#
# Check correctness.
#

# Rows and columns.
foreach my $x (0 .. $SN - 1) {
    my %hx;
    my %hy;
    foreach my $y (0 .. $SN - 1) {
        $hx {$grid -> [$x] [$y]} = 1;
        $hy {$grid -> [$y] [$x]} = 1;
    }
    die unless $SN == keys %hx && $SN == keys %hy;
}
    
# Grids.
foreach my $x (0 .. $N - 1) {
    foreach my $y (0 .. $N - 1) {
        my %hg;
        foreach my $xx (0 .. $N - 1) {
            foreach my $yy (0 .. $N - 1) {
                $hg {$grid -> [$x * $N + $xx] [$y * $N + $yy]} = 1;
            }
        }
        die unless $SN == keys %hg;
    }
}
             
# Print results (assume $N <= 8).
foreach my $x (0 .. @$grid - 1) {
    foreach my $y (0 .. @{$grid -> [$x]} - 1) {
        print $chars [$grid -> [$x] [$y]];
    }
    print "\n";
}

__END__
0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ
6789ABCDEFGHIJKLMNOPQRSTUVWXYZ012345
CDEFGHIJKLMNOPQRSTUVWXYZ0123456789AB
IJKLMNOPQRSTUVWXYZ0123456789ABCDEFGH
OPQRSTUVWXYZ0123456789ABCDEFGHIJKLMN
UVWXYZ0123456789ABCDEFGHIJKLMNOPQRST
123450789AB6DEFGHCJKLMNIPQRSTOVWXYZU
789AB6DEFGHCJKLMNIPQRSTOVWXYZU123450
DEFGHCJKLMNIPQRSTOVWXYZU123450789AB6
JKLMNIPQRSTOVWXYZU123450789AB6DEFGHC
PQRSTOVWXYZU123450789AB6DEFGHCJKLMNI
VWXYZU123450789AB6DEFGHCJKLMNIPQRSTO
23450189AB67EFGHCDKLMNIJQRSTOPWXYZUV
89AB67EFGHCDKLMNIJQRSTOPWXYZUV234501
EFGHCDKLMNIJQRSTOPWXYZUV23450189AB67
KLMNIJQRSTOPWXYZUV23450189AB67EFGHCD
QRSTOPWXYZUV23450189AB67EFGHCDKLMNIJ
WXYZUV23450189AB67EFGHCDKLMNIJQRSTOP
3450129AB678FGHCDELMNIJKRSTOPQXYZUVW
9AB678FGHCDELMNIJKRSTOPQXYZUVW345012
FGHCDELMNIJKRSTOPQXYZUVW3450129AB678
LMNIJKRSTOPQXYZUVW3450129AB678FGHCDE
RSTOPQXYZUVW3450129AB678FGHCDELMNIJK
XYZUVW3450129AB678FGHCDELMNIJKRSTOPQ
450123AB6789GHCDEFMNIJKLSTOPQRYZUVWX
AB6789GHCDEFMNIJKLSTOPQRYZUVWX450123
GHCDEFMNIJKLSTOPQRYZUVWX450123AB6789
MNIJKLSTOPQRYZUVWX450123AB6789GHCDEF
STOPQRYZUVWX450123AB6789GHCDEFMNIJKL
YZUVWX450123AB6789GHCDEFMNIJKLSTOPQR
501234B6789AHCDEFGNIJKLMTOPQRSZUVWXY
B6789AHCDEFGNIJKLMTOPQRSZUVWXY501234
HCDEFGNIJKLMTOPQRSZUVWXY501234B6789A
NIJKLMTOPQRSZUVWXY501234B6789AHCDEFG
TOPQRSZUVWXY501234B6789AHCDEFGNIJKLM
ZUVWXY501234B6789AHCDEFGNIJKLMTOPQRS
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Abigail

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