Procedures

Research & learn about gases and lifting
When a balloon is held by a string, the balloon is floating in a pool of air.  The balloon displaces an amount of air.  As long as the balloon is lighter than the air it displaces the balloon well float in air.  If other objects are attached to the balloon, the balloon must displace enough air to compensate for the weight of those objects as well.  When the balloon rises it has positive buoyancy. When the balloon floats in mid air, without moving up or down, it is neutrally buoyant.  I wanted to calculate the minimum amount of gas used, so my balloon should be just barely positively buoyant.

Study periodic table chart to pick gas for lifting
I studied the periodic chart  and learned that it goes from the lightest element  to the heaviest.  I found out that Hydrogen was the lightest element and thought it was a good gas to use.  I later learned that Hydrogen  was explosive so I did not use it.   I then looked at the second lightest element and that was Helium.  I used Helium because it was the second lightest element and is one of the relatively inert Noble gases (which means it doest easily combine with anything else or explode) It is available at grocery and party stores.

Research and calculate lifting power of gas
I looked at the periodic chart and found the  atomic weight and the atomic radius I was unable to calculate the lifting ability of Helium from these facts.  From a web page at Elmhurst collage I found that that Helium has a density about 0.18 grams per liter.  Air has  a density of  1.28 grams per liter. I subtracted those and determined that Helium would have a lifting ability of about 1.10 grams per liter.  I also found out that  Nitrogen weighs 1.2506 grams per liter and  makes up 80 % of the air we breath.  That is why the density in air is so close to the density of Nitrogen.

Measure weight of  object to be lifted
I used a "triple beam balance" to determine the weight of my mouse (10 grams).  I also weighed an empty balloon and string (9 grams).

Calculate amount of gas needed to provide lift
I calculated that 10 liters of Helium would lift 11 grams (1.10 grams/liter of lift x 10 liters = 11 grams). 
It should require about 0.9 liters of Helium for every gram to be lifted (11 grams / 10 liters = 0.9090 liters/gram)
I should need about 9 liters of Helium to float my mouse

Determine weight of balloons and string and add to weight to be lifted
The material of the balloons and the string or ribbon also have weight and must be considered too.  I weighed an empty balloon and string and found that they weighed 9 grams.

Determine amount of gas balloon could hold. 
I had planned on calculating this, but since the balloons were an irregular shape, I could not calculate it and had to use an empirical method for this.  I filled one of the balloons with water from a measuring cup and found that it held 10 liters.  I originally wanted to use round balloons, because I had a formula to calculate how much gas they would hold, based on their diameter.  However, I discovered that regular balloons are porous and would lose their Helium too quickly.  Instead I had to use Mylar balloons, which were not available in a shape that made it possible for me to calculate the amount of gas they could hold.

Predict number of balloons required
Based on the above calculations and empirical observations, I calculated that three balloons and the mouse would weigh 37 grams.  Since each balloon holds ten liters, three balloons would only lift 33 grams.  Therefore three balloons would not lift the mouse.  Four balloons would be required.

Test lift power one balloon to validate prediction
I then weighed the mouse once more and then attached one balloon.  The attachment of the balloon reduced the mouses weight by about 2.5 grams, which further supported my calculation of four balloons.

Attach balloons and lift mouse
I then began attaching balloons to test my analytical prediction.  I was very surprised to find that the mouse began to float after attaching only three balloons.

Resolve differences between analytical prediction and empirical results
Looking back at the process, I think that perhaps the method of filling the balloon with water to determine how much they could hold might not have been accurate enough.  Perhaps the balloons held slightly more than 10 liters. 

Also,  I was dependent on the information from Elmhurst college  as  to how much lift to expect from a liter of Helium.  It is possible their figure was slightly in error.

Lastly, the air temperature and pressure in the room may also have contributed.  If the temperature were lower, the Helium would have contracted and the balloons would have had less lift.  Perhaps the Elmhurst figures were based on a lower temperature.