In practice most small wood or GRP boats can virtually ignore deviation unless someone has mounted or left a ferrous object near the compass.
How to swing a compass and create a deviation card.
Preferably on a calm day in a location with little or no set.
Head out in your boat with your compass and a means of taking a bearing or azimuth. A good hand compass will do
A transit is good but not necessary.
Pick a conspicuous object at least 2 miles away if possible. ( the further away the less error if boats position changes slightly)
Starting with North
Turn slowly to steer North or 000 deg on compass. Compare heading with HBC and take azimuth or bearing of object and make a note of bearing.
Turn Slowly to E or 090 deg compass and repeat the tacking of bearing of object’
Turn slowly South. 180
Turn slowly West 270
Repeat the process turning slowly for NE 045
SW 225 deg
NW 315 deg
For example create a table
N 000 Deg Brg 150 deg Comp
NE 045 Deg brg 148 deg comp
E 090 deg Brg 146 deg comp
SE 135 deg Brg 144 deg comp
S 180 deg Brg 146 deg comp
SW 225 deg Beg 148 deg comp
W 270 deg Brg 150 deg com
NW 315 deg Brg 152 deg comp
Add all your bearings together and divide by 8
In this example it will equal 148 this is the compass bearing of the object
For this method variation is not required.
By applying the variation you would have the true bearing of the object.
You could do the same process with a known transit slowly turning and crossing the transit on each of the cardinal and inter cardinal points tacking the bearing each time you cross
It would be a very unusual vessel not to have symmetrical deviation card requiring no symmetrical metal or an compass off set to one side.
It is important to turn slowly between bearings so the compass does not lag or you will induce an apparent error into your deviation card.
Plot the results on a graph and join the dots and you have a deviation card.