What you will need or more to the point what I used, there is always room for improvisation.
Large block of green wood
Metal strapping; I used 20mm x 3mm stainless steel flat bar but anything of a similar size will do.
2 large coach bolts
Scraps of wood to rest and support main block while measuring
Grease for bolts
2 large nails
Laser Level (or spirit)
Drill and bit
Steel toe capped boots
Step 1: Measuring the Block
Of course no piece of wood that is in its raw state is a perfect shape. I wanted to make this into an eight sided block and also take maximum advantage of the size of the block. So I measured and marked out the geometry on the top of the block to make sure it was going to work and then found the centre of the top and base. I then stood the block on a piece of wood with a nail sticking up into the previously marked centre in the base. I then used a piece of wood supported between two benches with a nail again knocked into the centre mark on the top. The reason for doing this was so I could rotate the block around a central axis to mark the top and bottom so the two ends could be cut off perfectly parallel to each other to give me a perfectly level top for my anvil to sit on. The wooden block was also slightly asymmetrical and this allowed me to centre the two ends relative to each other. Once I was able to rotate the block I used a laser level to give me a stationary point and marked a horizontal line top and bottom. I also marked the vertical lines of each corner to give me guides for cutting the flat side faces of the octagon.
Step 2: Cutting the Ends
Now the wooden block was marked out I was able to take a chainsaw and cut off the ends so they were perfectly parallel and perpendicular to the central axis which meant the block would stand up straight and have a level top.
Step 3: Marking Out the Ends
Now the ends have been cut off nice and straight the block was ready to mark out again. I could still find the centre as I had driven the nail in far enough that the hole was still visible. I then used this and the vertical lines on the sides of the block to mark out the octagon on the top of the block.
Step 4: Cutting the Sides
Using a chainsaw I then cut down the sides. I did this on a slight angle to the vertical giving the block a slight taper. This is important for two reasons: firstly so the the steel bands to be fitted later can be knocked down giving the needed very tight fit. Secondly to make it look ascetically pleasing and give the appearance of stability with a broader base. The shape and size of the wood did not allow for much or a taper but enough to satisfy the above needs.
I then used an electric plane to give the sides a nicer finish and a chamfer on the top and bottom corners.
Step 5: Fitting the Steel Bands
The steel bands are essential to stop the block from splitting as it drys out. In this case I used green wood that was fresh cut as it is easier to work with. I used stainless steel 20 x 3 as it looks nice and was available. I measured the band and marked each corner then bent them to 135 deg. I then checked the band for fit, removed and then welded. Then I knock the bottom band on first. The band must be knocked down very hard bit by bit working your way around the block. The band must be as tight as possible to prevent the block from splitting.
Step 6: Finally Fitting the Anvil
Once I had checked that the block was sitting nice and steady and was perfectly level on top it was time to fit the Anvil. I marked out the holes, drilled them and screwed in two large bolts to hold it down Very tight. I applied a small amount of grease to the bolts to make them easier to screw in and to prevent rusting.
My anvil is now ready to use :-)