This is the example:
from /routing filter
0 ;;; Advertise 116.0.0.0/21 to OpenIXP in /24s
chain=to_OpenIXP prefix=116.0.0.0/21 prefix-length=24 invert-match=no action=accept

1 ;;; Advertise 61.45.224.0/20 to OpenIXP in /24s
chain=to_OpenIXP prefix=61.45.224.0/20 prefix-length=24 invert-match=no action=accept

2 ;;; Advertise only our prefixes to OpenIXP, do not redistribute transit routes to IX
chain=to_OpenIXP invert-match=no action=discard

3 ;;; Discard default route from OpenIXP
chain=from_OpenIXP prefix=0.0.0.0/0 invert-match=no action=discard

4 ;;; IX routes should get higher priority
chain=from_OpenIXP invert-match=no action=accept set-bgp-local-pref=200

5 ;;; Advertise 61.45.224.0/20 to NAP
chain=to_NAP prefix=61.45.224.0/20 prefix-length=20 invert-match=no action=accept

6 ;;; Advertise 116.0.0.0/21 to NAP
chain=to_NAP prefix=116.0.0.0/21 prefix-length=21 invert-match=no action=accept

7 ;;; Advertise only our prefixes to NAP, do not redistribute IX routes/other transit routes to this transit
chain=to_NAP invert-match=no action=discard

8 X ;;; Only use default route from NAP’s BGP feed, opposite the following rule
chain=from_NAP prefix=0.0.0.0/0 invert-match=yes action=discard

9 ;;; Discard default route from NAP
chain=from_NAP prefix=0.0.0.0/0 invert-match=no action=discard

from /routing bgp network
0 A 116.0.0.0/21 no
1 A 116.0.0.0/24 no
2 A 116.0.1.0/24 no
3 A 116.0.2.0/24 no
4 A 116.0.3.0/24 no
5 A 116.0.4.0/24 no
6 A 116.0.5.0/24 no
7 A 116.0.6.0/24 no
8 A 116.0.7.0/24 no
9 A 61.45.224.0/20 no
10 A 61.45.224.0/24 no
11 A 61.45.225.0/24 no
12 A 61.45.226.0/24 no
13 A 61.45.227.0/24 no
14 A 61.45.228.0/24 no
15 A 61.45.229.0/24 no
16 A 61.45.230.0/24 no
17 A 61.45.231.0/24 no
18 A 61.45.232.0/24 no
19 A 61.45.233.0/24 no
20 A 61.45.234.0/24 no
21 A 61.45.235.0/24 no
22 A 61.45.236.0/24 no
23 A 61.45.237.0/24 no
24 A 61.45.238.0/24 no
25 A 61.45.239.0/24 no

Please note that OpenIXP advertises the smaller prefixes (/24) to make sure that local IX traffic takes the OpenIXP path rather than the NAP path. I also choose to drop default route from NAP to receive full BGP feed from my NAP. If you wish to receive only the default route (make sure your NAP does provide a default route otherwise your packets will go nowhere), enable filter #8 and disable #9.

Good luck!