Put 25cm³ of the alkali of the known concentration (usually sodium hydroxide) into a conical flask using a glass pipette.
Fill a burette up to 0cm³ with your acid of unknown concentration.
Add some phenolphthalein indicator solution to your alkali. It will turn pink.
Put the burette on a steady drip, whilst swirling the conical flask around until the solution goes completely colourless.
Take the reading from the burette.
Now you can work out your concentration.