To tune a viola, you will need to have one of the following

• A tuner (either viola or chromatic tuner). You can find more information in the Tuners section.
• Perfect pitch (you can discern the pitch of a tone without the aide of any other device)
• A keyboard or a tuned piano (this is to give you a reference while tuning)
• A viola teacher

There are 2 ways to tune a viola: tuning pegs or fine tuners (also called string adjusters).  Tuning pegs are used for tuning when the string is at least a half note out of tune while the fine tuners are used when the string is less than a half note out of tune.  Fine tuners' tuning range is a lot smaller than the tuning pegs.  For more experienced players, you will find that their violas are not equipped with fine tuners on D, G or C strings; only A string is equipped with a fine tuner.  This is because A string is the most fragile and requires the most care during tuning. 

If 2-4 strings are very out of tune

You need to take extra care while tuning if all 2-4 strings are very out of tune.  

1. Gradually tighten each string in the sequence of C-G-D-A.  The reason for starting from C string is to prevent damaging the thinner A and D strings.
2. Tighten only 1-2 note higher on one string at a time.  Then tighten the next.  Do not tighten one string all the way to the desire pitch while leaving the other strings very loose.  This will create unequal pressure on the bridge causing it to fall.
3. Since the tuning will cause enough movement of the strings and sway the bridge, you need to make sure the bridge continues to stay perpendicular to the viola surface while being tuned.  Make sure the bridge is completely straight especially where it makes contact with the strings.  If it has become tilted, carefully make it straight.  The viola bridge is neither glued nor screwed onto the viola. It is the tension from the 4 strings that keeps the bridge in its place.
4. In order to keep the tuning pegs from slipping, you will need to apply inward pressure on the tuning pegs toward the viola head as you turn the tuning peg.  Tuning pegs are tapered hence the inward pressure will ensure that they stay in position once released.
5. If you have tuner, pluck the string and the tone will get registered on the tuner.  If you have a chromatic tuner, it will tell you what note you are on and how much further you have to go.  If you have a viola tuner, the tone will not register on the tuner until the tone is within the tuner's detection limit.  This is usually 1 note below or above the desired pitch.
6. Once you are within 1-2 notes from the desired pitch, you can use the fine tuners to adjust each string.
7. If you cannot tighten a fine tuner any more, loose the fine tuner half way first then bring the string closer to the pitch by using the tuning peg.  Then use the fine tuner again for the adjustment.  Depending on the design of the fine tuner, fine tuners that are tightened all the way are more likely to dent the viola top since the space between the viola top and the fine tuners is very small.
8. If you cannot loosen a fine tuner any more, tighten the fine tuner half way first then loosen the string closer to the pitch by using the tuning peg.  Then use the fine tuner again for the adjustment.

If strings are slightly out of tune

You should always follow the instruction above.  You may be able to tune the strings with just the fine tuners.  However, you should always check to make sure the bridge is still straight after tuning.