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.