In England and Wales it is installed by the private sector, the private organization that is contracted to deliver the service to the government.
Each day the court sends a fax to the private company to say who an electronic monitoring order has been made on, and the monitoring officer--or in some cases officers, because sometimes they go in twos—goes to the person's house to fit the tag by the end of that day. If it is an early release from prison, the prison plans the release dates and they notify the private company. The private company installs the equipment and fixes the tag on the day the prisoner comes out.
That's in England and Wales. In Sweden, a probation officer would do the installation.