How to install Marker Models 

step 1

Choose and download a marker model

step 2

Extract the file you downloaded

Step 3

When the file is extracted you will have a folder with 2 files. One is a picture of the marker(PNG FILE) and the other is the marker itself(MDL FILE). 

step 4

Now in order to determine which marker slot the marker you just downloaded will be assigned to, you must rename the file. Example: In order to assign it to the Intimidator slot you must rename it "v_timmy"

Here's a list of all the markers and what the file should be renamed to
Angel | v_angel
Bushmaster | v_bush
Autococker | v_cocker
E-Mag | v_emag
Excalibur | v_excal
Impulse | v_impulse
Tippmann 98 | v_m98
Mag | v_mag
Matrix | v_matrix
Orracle | v_orracle
Shocker | v_shocker
SL-68 | v_sl68
Spyder | v_spyder
Intimidator | v_timmy

step 5

The file now needs to be moved the DPB models folder. This folder can be found in the address below 
If you have 64bit Windows 
"C:\Program Files (x86)\Steam\steamapps\YOURSTEAMID\half-life\dpb\models"
If you have 32bit Windows 
"C:\Program Files\Steam\steamapps\YOURSTEAMID\half-life\dpb\models"
Once you move it it, a window will pop-up. You then press "Move and Replace" 

*NOTE* 
This will delete your old marker

Step 6

The new marker will now be installed. Go play some DPB now!