How to install blower motor relays??

Ok lets try this.
You can do this right at the blower so all that confusing wiring in the diagram does not matter.

The wire at the blower for the low speed position would get cut in two.
The switch end of that wire would connect pole 86 on the relay.
The heater end would get connected to pole 87 on the relay.

For the second blower speed, it's wire would get cut in two and the switch end gets connected to the same poles on the next relay and then continues on to the heater via the second speed wire.

The relays just act as a electrically operated switch that can handle a lot more amps that your dash switch.
All the dash switch would be doing after this is activating the relay for each speed, so the first relay would be for low, and the second relay would be for the second or high speed.

If it's a 3 speed blower, you just add another relay for that speed wired exacly the same as the others. (but using the 3rd speed wire.)

The wires (positive and negative) at the bottom of the diagram are what would now supply the blower with power and your dash switch would just be a light duty switch to activate the relays.

Keep in mind that a relay is only a electrically activated switch, that's all.
Terminals 85 and 86 make it switch on and off, and terminal 30 is where power comes in instead of going through your dash switch.
Terminal 87 on each relay is your blowers new power supply for each speed instead.

Ask if you need to.