Inhale it?! Oh my, no.
You're just going to want to drink it, make sure to mix it with a lot of juice or something because it's very corrosive and nasty tasting.
To measure it, you're just gonna need to get a pipette or blunt syringe, your local pharmacy should give these out for free.
For first dose, I wouldn't recommend more than 1ml. GBL can make you quite nauseous, especially on 1st time use. There's also some anecdotal evidence about how GBL doesn't really work properly the first couple of times you try it, and that it takes time to build up the enzymes or something (no idea the actual explanation for this), so be sure not to redose too much your first couple times, and work your doses up slowly. If your first dose of 1ml doesn't make you feel much, don't do more straight away, wait for it to wear off completely first (5 hours), then dose about 1.2ml and increase like that. Once you have some more experience using GBL, you'll be able to gauge how to dose more when already feeling the effect, but if you're a first time user, redosing will probably just result in you making yourself pass out.
Have fun

it can be an awesome drug, just be aware of its dangers, both in terms of OD and addiction
do you have any more questions, or have you already done your research on effects, duration etc.?