It's surprisingly easy. You need the construction set and a little time.

We'll be using scripts for this, and it's going to be a summon on equip effect.
As far as my know-how goes, it isn't especially hard to change from equip to cast.


To start, you need to create the creatures you want to summon. Easiest to use existing creatures and create new form id's after changing them.
If you want to summon one creature more than once, you will need multiple copies of it in the CS, each with a unique ID. ie.

AaSkeletonSummon
AaSkeletonSummon2
AaSkeletonSummon3

Changing the AI is important.
Open the creature window from the creature list (of the ones you wish to summon) and select AI.
Set the agression to 5, this will make it not attack everything in sight. Also, set responsibility to 0.
Delete any AI packages in the table at the bottom, and add a new one (right click in the table).
Set the ID to whatever you like and check these boxes: Must Complete, Defensive Combat. You may also check allow swimming and allow falls if you wish.
Change the package type to FOLLOW, and click on the target tab under the checkboxes. DO NOT CLOSE THIS WINDOW.

You will need a cell for your creatures. I recommend a custom cell which will not effect the base game.
The creatures need to be placed into the cell and then edited within the cell.
They need to have a unique reference (ideally similar to each other), need to be checked on persistant reference and on initially disabled.

Back in the AI package window, on the target tab, select specific reference.
Choose the cell you placed the creatures in, and select 'player''PlayerRef'
Press OK or save to close the window.


Right, time for some scripting.

######

ScriptName AaSkeleSummonScript
;Change the Script name.

;This script can be used as a template for custom summoned creatures/NPC's

float timer
float fade
short playonce
ref SUMN
ref SUMN2
ref SUMN3
;You will need to have one 'ref SUMNx' for each creature
;I have 3, so only need 3 'ref SUMNx'

Begin OnEquip

;Set temp reference for scripting.
;Allows faster transition of script as template. Also allows for leveled summon.
set SUMN to AaSkeletonRef
set SUMN2 to AaSkeletonRef2
set SUMN3 to AaSkeletonRef3
;This is where you bind each SUMN to a unique REF ID of the creature (found by opening creature window from cell)


;Now we move our creature to the Player
;Move the creature reference to the worldspace of the Player
;Leave the last number at 10, but change the others by 2

SUMN.moveto Player 32 32 10
SUMN2.moveto Player 28 28 10
SUMN3.moveto Player 30 30 10

;Make our creature visible and active

SUMN.enable
SUMN2.enable
SUMN3.enable

set fade to 1 ;Resets the alpha fade variable to 1
End

Begin OnUnEquip

SUMN.disable
SUMN2.disable
SUMN3.disable

End

######
Save the script and close it.


The next part is creating an item which the creatures will be bound to.
Items>Clothing is where the rings are, duplicate one and be sure to remove any existing enchantments. (You can use any equip-able item)

Okay, almost done.
Change the name, value and weight of the item, to what you like.
Make sure playable is checked, and probably quest item as well, to prevent you losing it by dropping/selling.
Open the drop tab for scripts and select the scriptname you chose.

Right, now we need to place the ring/other item into the world for you to find.
In the Cell View window (under the render) it is probably open with 'interiors'.
Change this to 'tamriel'
Click on 'location' to order it, will have -64,-29 at the top.
Scroll down to 11,20 and double click on it.

Zoom in to the world and place the ring.


Now, when you exit the sewers right at the start, you will have a ring that summons creatures for you.
Taking the ring off will instantly despawn them.


Please note, attacking these summoned creatures will make them attack you. You cannot yeild to all types and only unequipping and re-equipping will stop them attacking.