How do I create a filter variable and use it for selection?

open the “Data” tab and select “Filter” (either by using the symbol in the icon bar or the one in the bottom-left corner of the jamovi window)
in order to access functions, press the “fx” icon in the filter settings
there also is a switch where you can activate or deactivate the filter (see the comment in red below)
you close the filter settings by pressing the arrow in the top-right corner
there are three large approaches, to exclude outliers:
(1) based upon z-scores (the absolute value should be larger 3.3; this equals to a probability of 0.1% = 1 / 1000)
(2) based upon the IQR (like in a box plot)
(3) multivariate based on the Mahalanobis distance
for (1), there exists a function in jamovi (see next bullet point), for (2) and (3) you have to use R-code (decribed two bullet point below); for (2) you could also do it visually (three bullet points below)
you can either use an function-based selection (e.g., based on z-scores)
MAXABSZ([VARIABLE1], [VARIABLE2], …) < 3.3

you can manually create a select variable and use that (see below)
selSbj == 1
you can now use with R-scripts to find outliers based upon the interquartile range (doing the same with code that you visually do with box plots) and multivariate outliers based upon the Mahalanobis distance
the output from those scripts tells you which lines you should de-select
you use the scripts within the Rj editor, just copy-and-paste them and run them by hitting the "Play"-button (the little green triangle)
PLEASE NOTE: If you already filtered cases, the lines with filtered cases are not included and the line numbers when using the R-scripts get wrong.
Open the filter settings by either double-clicking on the header line of the filter variable or use the symbol in the bottom-left corner of the jamovi window (only to see if you selected the tab “Data”) and deactivate the filter.
the filter conditions can be combined using boolean and / or<:br>MAXABSZ([VARIABLE1], [VARIABLE2], …) < 3.3 and selSbj == 1
you can also de-select cases by setting limits to the values of variables (after visually checking the box-plots under “Descriptives”):
Revise < 50 and Anxiety > 51

How do I manually create a variable that I can use for selecting / deselecting cases?

open the “Data” tab so that you see your data spreadsheet
go to the first column (if there are filter variables, to the first column after those)
right-click on the header line in that column, choose "Add variable" in the menu that opens, and then Data Variable → "Insert"
briefly check how many participants are contained in your file by going to the last line of your data set, remember this line number

now, we have to to a little bit of magic in Excel, LibreOffice Calc or Google sheets: open one of these programs, write "1" in the first and the second line of an empty spreadsheet, mark these two lines and then use the little black marker bottom right to drag the ones until you reached the number of participants in your data set within jamovi
copy the column with the "1" you just created to jamovi and paste it into the variable you just created
open the filter settings (either by using the symbol in the icon bar or the one in the bottom-left corner of the jamovi window) and either change an existing filter with adding:
and [VARIABLE NAME] == 1
or create a new filter with just writing:
[VARIABLE NAME] == 1
obviously you have to adjust [VARIABLE NAME] to the name of your selected variable

now you can de-select case by changing them "1" to "0" (and also select them again by changing "0" into "1")