Tie: Wrists behind neck

Based on this tutorial.

1. Tie the wrists using e.g. Lark's Head Double Column Cuffs.
2. Bring the wrists over the head and pull down so that the tail of the rope is going down the centre of the back.
3. Take the tail of the rope off to one side and wrap around the front and back to the back.
4. Pull the tail between the back and the vertical rope and then down over the start of the wrap to form a cross.
5. Adjust the rope so that the cross is centered and the rope is tensioned as desired.
6. Wrap back again in the opposite direction (under or over the existing wrap).
7. Bring the tail through the bight formed in step 6.
8. Tie it off in a half-hitch on the opposite side.
9. Optionally use the spare rope to pull the wrists tighter.

If the wrists are not pulled far enough back, it is quite possible that the bottom can slip their head through an arm and get their elbows down. Pulling them back tightly may make this tie intolerable. Instead you can use more rope on the arms to make it impossible for the head to pass through.

Tie: Lark's Head Double Column Cuffs

Based on this tutorial

1. Form a lark's head and place it around both columns with the lark's head in the middle of the two columns.
2. Add 2 more wraps going up the columns so that there are 6 strands of rope around the columns. You now have 6 strands that go above and below the columns.
3. Pass the tail of the rope downwards through the bight formed by the lark's head 
4. Pass the tail of the rope under the lower strands of rope (going upwards relative to the columns)  then wrap it back over the top strands (going downwards).
5. Pass the tail of the rope downwards through the lark's head bight. This captures the entire upper and lower set of strands.
6. Adjust the tension to bring the columns together as much as desired.
7. Holding the knot under tension (if possible), open a bight downwards and on the opposite side from the lark's head then pass the tail over the top of the strands.
8. Pass the tail back under the upper strands to wrap them and pull them through the bight to form a half-hitch.
9. Readjust the tightness if needed.

Knot: Somerville Bow Line

1. Wrap the rope at least twice around the column (so 4 strands everywhere).
2. Leave about 10cm loose at the head.
3. Bring the tail over the top of the 4 strands.
4. Make a loop in the tail. The loop should pass under the tail. So now, the 4 strands and the loop are all laying under a section of the tail.
5. Take the head up through the loop and back across that section of the tail and feed it under the 4 strands coming up through the loop again. Probably easiest to reach 2 fingers through the loop and under the strands to grab it.
6. Tighten the loop.

The Somerville bowline is a secure knot but it is very hard to release it if the tail if under tension. You first have to ease the tension but that may be difficult. If needed, consider the following modification.

Slipped version

• At step 2: leave about 20cm at the head, instead of 10cm
• At step 5: above when feeding the head through the loop, fold it over so that you feed 4 strands through the loop and tighten on that. The head never goes through loop.
• When you need to release, pull the head. This will pull those 4 strands back out of the loop and the know will release.

Other notes

After tying, the head and tail will point in a opposite directions, perpendicular to the 4 strands. When you wrap the strands around the column, if you choose to put the head above the tail then at the end, the head will point down and the tail will point up and vice-versa.

The loop in step 4 can pass over the tail if you really want but then in step 5, you have to bring the head under the strands that are around the column but then into the loop from the top. Otherwise the loop will not form a knot. It's easier to form the loop this way but it's definitely harder to feed the head into it. Maybe it's useful sometimes...

Different technique is in this tutorial.