Since you already have an altar to her, this will be pretty easy. While at the altar, you can do one of many things. The only thing is, it might take some practice, patience, and time. 
The one I personally use is this one:
Instead of a sigil, I use the 
on a wooden circle, shown below:
The following two are from:
Invocation to Hekate of the Gates
Hekate
One Who Stands at the Gates
Lady of the Crossroads
Night-Wanderer
I invoke thee
Nocturnal one of the saffron robe
Torches in hand, you light the way
A choice to make at the crossroads
The darkness of the underworld to fare
Titaness, keeper of the keys of all the universe
Show me the way
Help me unlock the parts of myself that I have kept hidden
Help me unlock the path before me
That I may walk it
Not in fear, but with you at my side
Drawing in Hekate
I am older than they say
I am the conception of time
And will be there at the demise of all
I am the governor of all your crossroads
I am the footsteps along your path
And carry you when you can no longer walk
I am the brightest star in the night sky
I am the universe within
And the darkness without
I am magick in all things
I am poison of no things
And cure to life and transform death
Know me now by this name, Hekate
This one is from:
A simple chant to invoke Hecate, goddess of the crossroads and Queen of the Night:
Quiet is the night, Dark is the Moon
I ask Hecate, the Crone To take Her throne.
Sleep, sister, sleep, Be not alarmed,
For the Great Goddess Keeps you from harm.
Quiet is the night, Dark is the Moon
I ask Hecate, the Crone To take Her throne.
Sleep, brother sleep, Be not alarmed,
For the Great Goddess Holds you in her arms.
Quiet is the night Dark is the Moon.
We honour Hecate the Crone Who has taken Her throne.”