If you'd like to include links to other journal entries, follow these steps:
Tap the Date at the top or the Map at the bottom of an entry to access the Metadata view.
Tap Entry ID.
In the sheet that appears, you can choose from three options: Copy Entry URL, Copy Markdown Link, and Copy Entry ID. Tap Copy Markdown Link. (For including a link in an entry, this is the best option.)
Open another entry and tap where you would like to place the cursor. Paste the link in this entry by long-pressing (press and hold a finger on the spot where you want to add your link), then tap Paste.
Dismiss the keyboard by tapping the down arrow at the top left of the keyboard or swipe down on the keyboard.
Version 3 or higher
Click the More icon (•••) at the top right of an entry
Click Share > Copy Entry URL
Paste that into the desired entry
Right-click (Control+click) any entry in the Timeline
Choose Share > Copy Entry URL
Paste into the desired entry
Version 2.9.2 and earlier
In the Timeline, reveal the contextual menu by control-clicking (or right-click) the entry you want to link to.
Select Copy Markdown Link.
Open another entry in Edit mode. Paste the link in this entry. If you like, you can modify the text of the link by changing the text in brackets.