Coating the lash and ensuring a good bond
You do not need to coat the entire lash in the adhesive to get a good bond. You will actually find that using too much glue will just leave you with an excessive amount of stickies and you will spend a good amount of the treatment time pulling them apart. When you first train you will be advised to use a sensitive glue. This is obviously good as it helps you get used to attaching the lash but I recommend advancing to an intermediate adhesive as soon as possible. You will find that with sensitive adhesives, the setting time is a lot slower so lashes are more likely to stick together and the retention will not be as good. The quicker setting your adhesive, the quicker you will advance in perfecting your technique.
Things you will need to consider when lashing...
The humidity & temperature of your room.
The ideal environment for your chosen adhesive.
How quickly you can apply the lashes.
Humidity and temperature are such an important influencing factor when lashing and there is another page on the topic so I will only briefly touch on it here. The higher the humidity, the faster your adhesive is going to set so you may want to grab a little more adhesive so that it is not set before it reaches the lash. Faster swiping into the adhesive will pick up more and you will be able to see the bead. If you swipe your lash into the glue a little slower you will get more seamless coverage but it will set really quickly.
The adhesive you choose will also make a difference to how you lash. For fast setting adhesives you will need to be very quick at isolating and application you will want to be picking up a little more adhesive. If at any point you pick up a bit too much adhesive you can dab off any excess onto the eye pad beneath (but don't make a habit of it). I don't like to do this as it can contribute to over exposure for the client but you could do it whilst you get used to the amount of adhesive you will need. Alternatively you could shimmy the synthetic lash just the tiniest bit when you bond it, but do not lift away as your retention will suffer.
If you don't apply the lash at the correct angle you may find that it is only the tip or middle of the lash rather than the base that bonds and they will easily come apart, especially when brushing as the base will catch on the brush.
Ensuring a good bond
When I trained I was taught to swipe the synthetic lash over the natural one several times to spread the adhesive but I find that you will get better retention if you just place your lash on top. Swiping the lash exposes the adhesive and thins it out so I don't think it gives the best bond. If you place, the adhesive will envelop the natural lash and the bond will be anchored in place.
As I said earlier, you do not need to cover the lash in adhesive. Ideally you will only want about 2mm covered. To get the best retention you need to make sure that it is the bases that are securely bonded. You will need to ensure that your bond is seamless and following the angle of the natural lash will help you to achieve this. If you approach with your lash at just the right angle you will find that the lashes will bond perfectly. It should be a fluid movement and if all the conditions are right you will find that the bonding effect will seem almost magnetic. In the diagrams below you can see the importance to applying the lash at the right angle.