Where should I mount relays on an electronic circuit board?
The 833H type of relays are designed to be PCB mounted. Since they are not so large they will work well on standard 62.5 mil thick boards. 

When determining where to place them relative to the other circuitry on the board you need to consider a number of factors:

1) Locate so that the relay contact terminals are near to to the connector or terminals that will be used to wire your board to the externally power switched equipment.

2) Use care to mount larger components like this near to a board mounting hole to provide stability and lower the amount of board flexure the the product is deployed into service. 

3) Since you will be switching a high voltage it is necessary that you layout the board in a manner to provide proper spacing and isolation between any pins / pads / traces of the high voltage part of the circuit and the the low voltage portion.

4) Consideration also needs to be made as to how the board will be mounted so that high voltage nodes on the the board are not accessible to a user when the unit is placed into use. This may require a special case, insulation layers or shields around the relay area of the board.