This assumes you are connected to your XenServer with XenCenter.  First you need to mount the xs-tools.iso on the system.  To do this right click on the VM you want to install XenServer Tools on and click “Install XenServer Tools”

Then from the console tab do the following as the root user, or just type sudo before each command

mkdir /mnt/xs-tools

mount /dev/cdrom /mnt/xs-tools

cd /mnt/xs-tools/Linux/

bash install.sh

After it finishes, you need to reboot.