Carefulness costs you nothing. Carelessness may cost you your life.
When applying mechanical advantage (MA), the rope and knots are as important as the pulleys.
You will need more rope to make more force. For example, a 5:1 mechanical advantage will need five feet of line for every one foot gained.
Anytime a knot is tied in a rope, the rope is weakened. In drop tests and pull tests, a rope typically breaks at the knot. Attaching the pulleys to the pull line with midline knots reduces strength. A better option is to use a separate Prusik cord or a hitch such as the Valdotain Tresse Knot (VT). This retains rope strength while allowing adjustment. Phillip Kelley will show you how to do a modified VT in the video below.
In MA systems, there should be some form of control in case the pulleys meet and become two-blocked. The load must to be held in place until a new pull can be made. Attaching the pulleys with a Prusik cord will hold the load while attaching the other end of the MA system using a separate rope and tying a VT allows for one pulley to be slid away from the other. The video below demonstrates this.
Valdotain Tresse Knot for Climbing and Rigging
Phillip Kelley of North American Training Solutions demonstrate a fast, easy VT that you can use to tie on to the pull line.