Mastering the Pen Tool in Adobe Illustrator
Prepare
Learn to work with two hands, one to move the mouse, one to press the Spacebar, Shift, Tab, Alt/Option and Command/Control keys.
Activate “Snap to point” from the “View” menu.
With Cmd/Ctrl-U, pink intelligent guidelines appear that can help with aligning the next point. Often this is annoying. Memorize the shortcut.
Use Cmd/Ctrl-Y to switch between Preview and Outline mode. Basic knowledge.
Go to Preferences > Selection & Anchor Display and activate [V] Constrain Path Dragging on Segment Reshape, the fourth checkbox from the top. That works better to fine-tune curves. Many other options on this page are related to the pen tool, try them out some time.
With the Selection Tool (V), a path can be selected. With the Direct Selection Tool (A), one or more points on a path can be selected. In Illustrator a selected point is dark (filled) and non-selected points are hollow.
To digitize letter shapes from a photo, set Fill to “None”. Set Stroke to a thin line in a color that contrasts well with the colors in the photo.
Put photos or scans in a separate layer and lock it.
All paths are drawn with on-curve Anchor points. Curved segments have additional off-curve Handles that control the curvature.
Anchor points in Illustrator can be Corner points or Smooth points. For Smooth points, the Handles on either side of the point have the same direction. Illustrator does not have handy tangent points that you might find in other drawing software.
First choose the Direct Selection Tool (A), and then the Pen Tool (P). Pressing Cmd/Ctrl then gives access to the previously selected tool, Direct Selection, which is practical.
Start, Straight lines first
At first, the Pen Tool has a small asterisk (*) next to it, to symbolize the birth of a new path.
Click on the canvas to set a point, click once more to create a straight line.
Press and hold the Shift key to constrain the placement of the next point to 45 degrees angles in relation to the previous point.
You are now creating an open path with two endpoints.
In the active path, click on a previously set point to remove that point from the path. The Pen cursor shows a minus (-).
Click in between two points on the path to insert an extra point. The Pen cursor shows a plus (+).
Curved segments
A curved segment has two handles by definition, a straight segment should have no handles. When one of the handles of a curved segment is on the point it originates from, the curved segment seems to have only one handle. Segments with one visible handle are easier for very shallow curves but are mathematically not ideal.
As usual, first choose the Direct Selection Tool (A), and then the Pen Tool (P).
Click, hold down the mouse button and drag. This draws smooth curve points with control vectors on either side of the point, the handles are of equal length and direction.
Click-Drag again to finish the first curved segment and start the next.
Pressing the Shift key before clicking constrains the position of the next point, pressing the Shift key while dragging constrains the direction of the handle to 45-degree angles.
Click-drag again on the last smooth point to differentiate the length of the outgoing handle, the point remains smooth. or simply press Cmd/Ctrl while dragging. I love this relatively new improvement.
The power of the Option/Alt key
Pressing the Alt/Option key while dragging a handle enables a different length and direction for the outgoing handle, the smooth point becomes a corner point.
Press the Alt/Option key to quickly access the Anchor Point Tool (Shift+C). It looks like an upside-down V.
Alt/Option-click on a smooth point in a selected path to retract both handles coming from this point. Room for improvement: Alt/option-click on a handle to retract just this handle
Alt/Option-click-Drag on a corner point in a selected path to drag out handles of equal length.
Alt/Option-click-drag on the handle of a smooth point to move it independent of the other handle. The smooth point becomes a corner point.
With the Alt/Option key pressed, (or with the Cmd/Ctrl key pressed) click on a curved segment to reshape the segment directly. The cursor becomes a black arrow with a small curve segment next to it. Constrain Path Dragging on Segment Reshape should be switched on in preferences, and smart guides should be off (Cmd/Ctrl-U).
(Only when using Alt/Option-drag on a straight segment to turn it into a curved segment, it is more practical when the option “Constrain Path Dragging on Segment Reshape” is switched off. This unfinished Adobe business, I suggest considering converting a straight segment into a curved segment using the Alt/Option key a special case that should never constrain the direction of the newly born handles, regardless of the setting in preferences.)
Alternating straight and curved segments
When a curved segment should be followed by a straight one, click on the last point to remove the outgoing handle, (the cursor shows an upside-down v) and click somewhere else to add the straight segment.
When a straight segment should be followed by a curved one, click-drag from the last drawn corner point. (the cursor shows an upside-down v) Room for improvement: this new handle could be in the direction of the previous straight-line segment by default (like triangle tangent points in font editing software) unless or when Alt/Option is pressed.
Closing paths
In font production, paths are closed, in Illustrator they don’t have to be.
While drawing a path, click on the start point of the path to close it. The cursor shows a small circle (o). When the closing point is smooth, click-drag modifies the length of the last handle of the previous curve segment plus the direction of first handle of the path.
Press Cmd/Ctrl to temporarily access the Direct Selection Tool, select two open end points by clicking or dragging over them, and type Cmd/Ctrl-J (Join). This will draw a straight-line segment between the two points.
When you move the two endpoints of a path on top of each other (Snap to point must be active) the path will not be closed. Select the double point with the Direct Selection tool and type Cmd/Ctrl-J to join.
To open a closed path, click on it with the Scissors Tool (C)
Be in Command/Control
Press Cmd/Ctrl to temporarily switch to the Direct Selection Tool, to move any point on the path you are drawing.
Press Cmd/Ctrl (or Alt/Option) to modify the curvature of a curved segment. Activate “Constrain Path Dragging on Segment Reshape” and deactivate smart guides (Cmd/Ctrl-U) for better control.
Press Cmd/Ctrl to move a straight segment within a path to a new position. The handles of adjacent curved segments stay in place, wow. (this was default behavior in the great FontStudio from 1991)
To start a new path, press Cmd/Ctrl and click in an empty area to deactivate the previous path and then continue drawing.
Press Cmd/Ctrl to select one or more points with the mouse. With the arrow keys of the keyboard, selected point(s) can be moved around, according to Preferences > General > Keyboard Increment. I set it to a very small value. With the Shift key pressed, this value is multiplied by 10.
Press command near a corner might show controls to add rounded corners.
Continuing the path
Illustrator will show you where the line goes before you set the next point. After using the arrow keys, the Pen Tool loses connection with the active path (I think this is a bug) and you will have to click on an endpoint of a path to continue drawing from there.
When the drawing was interrupted, or if you want to continue drawing from the other side of the path, press Ctrl/Cmd to deselect and then reselect the path. Then use the pen tool to click or click-drag on any endpoint of the path to continue drawing from there.