It is important that you first understand how the pre-emergent herbicide works. There are many ways to apply this type of herbicide.
Pre-emergent herbicide information
A pre-emergent herbicide must be used when the weeds haven’t germinated yet, usually around spring time.
There are many ways to do this but the most common one are using a spray bottle or using a lawn spreader. You can buy a pre-mixed spraying herbicide from any gardening shop. Just remember where weeds were last season and apply the herbicide there. If you successfully found the places where the weeds were sprouting, then they should not grow this year. The herbicide can be used also after pulling out small weeds. Just spray on the remains of the weed to prevent it from growing again. Weeds with stronger roots can be killed by pulling out the little ones around them and then applying the weed killer.
You can also prevent weeds from growing by using granules. You can either spread them on the lawn or combine them with lawn fertilizers to create a double effect solution. After spreading the granules in any way you wish, turn on the sprinkler or water the lawn manually to activate the granule herbicide.
Later in the year
If the weather has started to get warmer, switch from pre-emergent herbicide to post-emergent herbicide and target existing, already grown weeds. Apply this type of herbicide a few times over the summer time. In autumn, switch back to pre-emergent herbicide and apply it on the lawn to prevent weeds appearing in spring.